AVP 26 not adding vendor ID

When using AVP type 26 you are supposed to also add the vendor ID. This code does not do that it sends in the packet the AVP type, the length and then right away the payload. For example if the AVP is 26 and the AVP length is 40 then in a trace we should see a HEX of:
"1a 28 00 00 00 09" + Payload. With this code we see:
"1a 22" + Payload