Just in case someone else is running into this issue, I hope I will save you some grief. After I upgraded my vsphere environment to 6 recently, I decided to separate my vmotion vmkernel ports onto a separate TCP/IP stack, which has been supported since 5.5 I believe, but it's easier now (GUI accessible). Anyway, I noticed that particular vmk was not showing up on solarwinds polls. I raised a support ticket and did an SNMPwalk, and was surprised to find that there was no information about that vmk in the SNMP poll, so Solarwinds support wasn't going any further. After that I opened a ticket with vmware and they basically said "here's the OIDs we support, if it's not working it's a feature request".
Long story short - if you separate any vmk's out onto separate tcp/ip stacks in esxi, you cannot monitor those ports with Solarwinds. I did open a feature request with vmware, but I won't hold my breath.