John, it sounds like you may be mixing up the rules for USVTA and USGT. Let me explain...
Here is the link to the official USVTA and USGT rules. Note that the USVTA rules are near the top of the page. The USGT rules are below.
http://www.usvintagetransam.com/rules/index.html
Generally speaking, these are great rules to follow and I support them. Yes, there is a spec tire for USVTA, but it's the same tire we've used for a long time. It's also in stock and readily available. USGT just started using a spec tire (for next year) and it's sometimes out of stock, but quickly replenished.
Here's where things can get a bit confusing.... ROAR also has a VTA class (referred to as "ROAR VTA"). Basically the rules are identical to USVTA rules with a couple exceptions: Any ESC is legal provided it is in "blinky" mode. And any 25.5T ROAR approved motor is legal. Someone can correct me, but I believe those are the only rules differences.
So if you're legal for USVTA, you're legal for ROAR VTA.....but not necessarily the other way around. Here in Austin we're "unofficially" running ROAR VTA I suppose. So we have some people running high end ESCs. It's quite likely they have a slight competitive advantage because of this, but their electronics aren't legal if they go to a track that runs USVTA rules. And this is the reason I just suggest people run USVTA legal equipment.
Good luck and hopefully we can race together soon! It's a fantastic class!
Tommy