What does VX stands for?

15

VX is probably just a designation given to something like a car, such as GT (Grand Touring)

It doesn't mean '15'. If it did mean 15 in Roman Numerals, it would read XV.

X = 10 and V = 5. It would never be written the other way round. As in VX Poison Gas, it means Vilaborate Xetan or Poison Twenty.