I have no LUA error to provide, but rarely, upon opening a vendor, my entire UI disappears and I have to do /reload for it to come back. I'm not entirely certain this is a Spartan issue, but... maybe?

I have the same issue. Most of the time it is happening when doing the Mrrl trade game. But having it in other places too. Just now I stand in Dazalor on my Yak Mount and opened the vendor. You can do still everything but you will not see anything until reload.