We actually ended up getting our issue resolved with support & some of the product folks (not sure if you're the same Sachin that was there too, haha).
Long story short, it appeared that the upgrade from vShield Manager 5.1.1 to 5.1.2 changed how the VXLAN deployment works as far as utilizing VUM or not for the VIB installs to the ESXi hosts. 5.1.1 would utilize VUM for pushing the VIBs to the hosts, while 5.1.2 changed this behavior to instead bypass VUM. Ironically, the reason for this change was to fix this very issue (VIBs not getting installed/VMKnics not getting created), yet changing a MOB config to essentially go back to the 5.1.1 behavior of going through VUM fixed this for us. Go figure.
Our only concern is that we haven't received the necessary documentation from support on where that actual MOB config was located at in vCenter for changing this behavior back to the 5.1.1 style behavior of going through VUM. We were told we were going to get at least some kind of rudimentary documentation for that prior to our ticket being closed, but the ticket was closed without any documentation sent to us.