diff --git a/src/dhcp.c b/src/dhcp.c
index 11c69bc1..e2d48244 100644
--- a/src/dhcp.c
+++ b/src/dhcp.c
@@ -808,7 +808,6 @@ make_message(struct bootp **bootpm, const struct interface *ifp, uint8_t type)
 	const struct dhcp_lease *lease = &state->lease;
 	char hbuf[HOSTNAME_MAX_LEN + 1];
 	const char *hostname;
-	const struct vivco *vivco;
 	int mtu;
 #ifdef AUTH
 	uint8_t *auth, auth_len;
@@ -1137,35 +1136,35 @@ make_message(struct bootp **bootpm, const struct interface *ifp, uint8_t type)
 		       p += ifo->mudurl[0] + 1;
 		}
 
+#ifndef SMALL
 		if (ifo->vivco_len &&
 		    !has_option_mask(ifo->nomask, DHO_VIVCO))
 		{
-			AREA_CHECK(sizeof(ul));
-			*p++ = DHO_VIVCO;
-			lp = p++;
-			*lp = sizeof(ul);
-			ul = htonl(ifo->vivco_en);
-			memcpy(p, &ul, sizeof(ul));
-			p += sizeof(ul);
-			for (i = 0, vivco = ifo->vivco;
-			    i < ifo->vivco_len;
-			    i++, vivco++)
-			{
-				AREA_FIT(vivco->len);
-				if (vivco->len + 2 + *lp > 255) {
-					logerrx("%s: VIVCO option too big",
-					    ifp->name);
-					free(bootp);
-					return -1;
-				}
-				*p++ = (uint8_t)vivco->len;
-				memcpy(p, vivco->data, vivco->len);
-				p += vivco->len;
+			struct vivco *vivco = ifo->vivco;
+			size_t vlen = ifo->vivco_len;
+			struct rfc3396_ctx rctx = {
+				.code = DHO_VIVCO,
+				.buf = &p,
+				.buflen = AREA_LEFT,
+			};
+
+			for (; vlen > 0; vivco++, vlen--) {
+				ul = htonl(vivco->en);
+				if (rfc3396_write(&rctx, &ul, sizeof(ul)) == -1)
+					goto toobig;
+				lp = rfc3396_zero(&rctx);
+				if (lp == NULL)
+					goto toobig;
+				if (rfc3396_write_byte(&rctx,
+				    (uint8_t)vivco->len) == -1)
+					goto toobig;
+				if (rfc3396_write(&rctx,
+				    vivco->data, vivco->len) == -1)
+					goto toobig;
 				*lp = (uint8_t)(*lp + vivco->len + 1);
 			}
 		}
-
-#ifndef SMALL
+		
 		if (ifo->vsio_len &&
 		    !has_option_mask(ifo->nomask, DHO_VIVSO))
 		{
diff --git a/src/dhcp6.c b/src/dhcp6.c
index ca076e4e..ea0bff53 100644
--- a/src/dhcp6.c
+++ b/src/dhcp6.c
@@ -276,29 +276,28 @@ dhcp6_makeuser(void *data, const struct interface *ifp)
 	return sizeof(o) + olen;
 }
 
+#ifndef SMALL
+/* DHCPv6 Option 16 (Vendor Class Option) */
 static size_t
 dhcp6_makevendor(void *data, const struct interface *ifp)
 {
 	const struct if_options *ifo;
-	size_t len, vlen, i;
+	size_t len = 0, optlen, vlen, i;
 	uint8_t *p;
 	const struct vivco *vivco;
 	struct dhcp6_option o;
 
 	ifo = ifp->options;
-	len = sizeof(uint32_t); /* IANA PEN */
-	if (ifo->vivco_en) {
-		vlen = 0;
+	if (ifo->vivco_len > 0) {
 		for (i = 0, vivco = ifo->vivco;
 		    i < ifo->vivco_len;
 		    i++, vivco++)
-			vlen += sizeof(uint16_t) + vivco->len;
-		len += vlen;
+			len += sizeof(o) + sizeof(uint32_t) + sizeof(uint16_t) + vivco->len;
 	} else if (ifo->vendorclassid[0] != '\0') {
 		/* dhcpcd owns DHCPCD_IANA_PEN.
 		 * If you need your own string, get your own IANA PEN. */
 		vlen = strlen(ifp->ctx->vendor);
-		len += sizeof(uint16_t) + vlen;
+		len += sizeof(o) + sizeof(uint32_t) + sizeof(uint16_t) + vlen;
 	} else
 		return 0;
 
@@ -312,19 +311,19 @@ dhcp6_makevendor(void *data, const struct interface *ifp)
 		uint16_t hvlen;
 
 		p = data;
-		o.code = htons(D6_OPTION_VENDOR_CLASS);
-		o.len = htons((uint16_t)len);
-		memcpy(p, &o, sizeof(o));
-		p += sizeof(o);
-		pen = htonl(ifo->vivco_en ? ifo->vivco_en : DHCPCD_IANA_PEN);
-		memcpy(p, &pen, sizeof(pen));
-		p += sizeof(pen);
 
-		if (ifo->vivco_en) {
+		if (ifo->vivco_len > 0) {
 			for (i = 0, vivco = ifo->vivco;
 			    i < ifo->vivco_len;
-			    i++, vivco++)
-			{
+			    i++, vivco++) {
+				optlen = sizeof(uint32_t) + sizeof(uint16_t) + vivco->len;
+				o.code = htons(D6_OPTION_VENDOR_CLASS);
+				o.len = htons((uint16_t)optlen);
+				memcpy(p, &o, sizeof(o));
+				p += sizeof(o);
+				pen = htonl(vivco->en);
+				memcpy(p, &pen, sizeof(pen));
+				p += sizeof(pen);
 				hvlen = htons((uint16_t)vivco->len);
 				memcpy(p, &hvlen, sizeof(hvlen));
 				p += sizeof(hvlen);
@@ -332,17 +331,22 @@ dhcp6_makevendor(void *data, const struct interface *ifp)
 				p += vivco->len;
 			}
 		} else if (ifo->vendorclassid[0] != '\0') {
+			o.code = htons(D6_OPTION_VENDOR_CLASS);
+			o.len = htons((uint16_t)len);
+			memcpy(p, &o, sizeof(o));
+			p += sizeof(o);
+			pen = htonl(DHCPCD_IANA_PEN);
+			memcpy(p, &pen, sizeof(pen));
+			p += sizeof(pen);
 			hvlen = htons((uint16_t)vlen);
 			memcpy(p, &hvlen, sizeof(hvlen));
 			p += sizeof(hvlen);
 			memcpy(p, ifp->ctx->vendor, vlen);
 		}
 	}
-
-	return sizeof(o) + len;
+	return len;
 }
 
-#ifndef SMALL
 /* DHCPv6 Option 17 (Vendor-Specific Information Option) */
 static size_t
 dhcp6_makevendoropts(void *data, const struct interface *ifp)
@@ -875,10 +879,10 @@ dhcp6_makemessage(struct interface *ifp)
 
 	if (!has_option_mask(ifo->nomask6, D6_OPTION_USER_CLASS))
 		len += dhcp6_makeuser(NULL, ifp);
-	if (!has_option_mask(ifo->nomask6, D6_OPTION_VENDOR_CLASS))
-		len += dhcp6_makevendor(NULL, ifp);
 
 #ifndef SMALL
+	if (!has_option_mask(ifo->nomask6, D6_OPTION_VENDOR_CLASS))
+		len += dhcp6_makevendor(NULL, ifp);
 	if (!has_option_mask(ifo->nomask6, D6_OPTION_VENDOR_OPTS))
 		len += dhcp6_makevendoropts(NULL, ifp);
 #endif
@@ -1199,10 +1203,10 @@ dhcp6_makemessage(struct interface *ifp)
 
 	if (!has_option_mask(ifo->nomask6, D6_OPTION_USER_CLASS))
 		p += dhcp6_makeuser(p, ifp);
-	if (!has_option_mask(ifo->nomask6, D6_OPTION_VENDOR_CLASS))
-		p += dhcp6_makevendor(p, ifp);
 
 #ifndef SMALL
+	if (!has_option_mask(ifo->nomask6, D6_OPTION_VENDOR_CLASS))
+		p += dhcp6_makevendor(p, ifp);
 	if (!has_option_mask(ifo->nomask6, D6_OPTION_VENDOR_OPTS))
 		p += dhcp6_makevendoropts(p, ifp);
 #endif
diff --git a/src/if-options.c b/src/if-options.c
index c50f65a1..54ae590b 100644
--- a/src/if-options.c
+++ b/src/if-options.c
@@ -656,6 +656,7 @@ parse_option(struct dhcpcd_ctx *ctx, const char *ifname, struct if_options *ifo,
 	struct dhcp_opt **dop, *ndop;
 	size_t *dop_len, dl, odl;
 	struct vivco *vivco;
+	const struct vivco *vivco_endp = ifo->vivco + ifo->vivco_len;
 	struct group *grp;
 #ifdef AUTH
 	struct token *token;
@@ -2111,6 +2112,10 @@ parse_option(struct dhcpcd_ctx *ctx, const char *ifname, struct if_options *ifo,
 		break;
 	case O_VENDCLASS:
 		ARG_REQUIRED;
+#ifdef SMALL
+			logwarnx("%s: vendor options not compiled in", ifname);
+			return -1;
+#else
 		fp = strwhite(arg);
 		if (fp)
 			*fp++ = '\0';
@@ -2119,6 +2124,12 @@ parse_option(struct dhcpcd_ctx *ctx, const char *ifname, struct if_options *ifo,
 			logerrx("invalid code: %s", arg);
 			return -1;
 		}
+		for (vivco = ifo->vivco; vivco != vivco_endp; vivco++) {
+			if (vivco->en == (uint32_t)u) {
+				logerrx("vendor class option for enterprise number %u already defined", vivco->en);
+				return -1;
+			}
+		}
 		fp = strskipwhite(fp);
 		if (fp) {
 			s = parse_string(NULL, 0, fp);
@@ -2149,11 +2160,12 @@ parse_option(struct dhcpcd_ctx *ctx, const char *ifname, struct if_options *ifo,
 			return -1;
 		}
 		ifo->vivco = vivco;
-		ifo->vivco_en = (uint32_t)u;
 		vivco = &ifo->vivco[ifo->vivco_len++];
+		vivco->en = (uint32_t)u;
 		vivco->len = dl;
 		vivco->data = (uint8_t *)np;
 		break;
+#endif
 	case O_AUTHPROTOCOL:
 		ARG_REQUIRED;
 #ifdef AUTH
@@ -2994,12 +3006,12 @@ free_options(struct dhcpcd_ctx *ctx, struct if_options *ifo)
 	    opt++, ifo->dhcp6_override_len--)
 		free_dhcp_opt_embenc(opt);
 	free(ifo->dhcp6_override);
+#ifndef SMALL
 	for (vo = ifo->vivco;
 	    ifo->vivco_len > 0;
 	    vo++, ifo->vivco_len--)
 		free(vo->data);
 	free(ifo->vivco);
-#ifndef SMALL
 	for (vsio = ifo->vsio;
 	    ifo->vsio_len > 0;
 	    vsio++, ifo->vsio_len--)
diff --git a/src/if-options.h b/src/if-options.h
index f3b9c17a..c892e75d 100644
--- a/src/if-options.h
+++ b/src/if-options.h
@@ -61,7 +61,6 @@
 #define USERCLASS_MAX_LEN	255
 #define VENDOR_MAX_LEN		255
 #define	MUDURL_MAX_LEN		255
-#define ENTERPRISE_NUMS_MAX_LEN	255
 
 #define DHCPCD_ARP			(1ULL << 0)
 #define DHCPCD_RELEASE			(1ULL << 1)
@@ -220,12 +219,12 @@ struct if_ia {
 #endif
 };
 
+#ifndef SMALL
 struct vivco {
+	uint32_t en;
 	size_t len;
 	uint8_t *data;
 };
-
-#ifndef SMALL
 struct vsio_so {
 	uint16_t opt;
 	uint16_t len;
@@ -303,13 +302,12 @@ struct if_options {
 	size_t nd_override_len;
 	struct dhcp_opt *dhcp6_override;
 	size_t dhcp6_override_len;
-	uint32_t vivco_en;
-	struct vivco *vivco;
-	size_t vivco_len;
 	struct dhcp_opt *vivso_override;
 	size_t vivso_override_len;
 
 #ifndef SMALL
+	size_t vivco_len;
+	struct vivco *vivco;
 	size_t vsio_len;
 	struct vsio *vsio;
 	size_t vsio6_len;