I'm in the process of gluing up a set of new HPI VTA tires on new HPI VTA wheels (white 8-spoke). It has been a while since I've glued up tires, so I don't think there is as much of a difference between the sidewall diameter as compared to the new Protoform tire. However there is definitely a difference between the tread pattern.
Here is a picture of the HPI 31mm VTA tire that I just glued today (8/17//2017)
Below is a picture of the Protoform HPI 31mm VTA tire from Protoform's website.
As you can see when comparing the 2 tires that the gaps between the treads do appear to be different.
HPI VTA tire - gap is smaller
Protoform VTA tire - gap is wider
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There may be a simple reason for the difference, such as the HPI molds being more worn out. Or it may be possible that the Protoform VTA tire needed to have a slightly different tread pattern to avoid any copyright infringement.
I'm only posting this as something I've noticed (that others may have noticed as well). Whether the actual tread pattern has any affect on performance will probably be irrelevant. Based on my own experience, I've always had to slightly adjust my setup/traction compound routine depending on how new or old the VTA tires I've been using. I've raced VTA on many different surfaces and have been beat by good drivers running new tires, old tires, ballooned tires, and almost balding tires.
In the end the driver with the best setup/driving has almost always won the main. The majority of the A-main driver's fast laps where always within a tenth. And lastly the cars have always seemed fairly equal going down the long straights.