Cash/Comps
There are some VP games that actually over an edge on the machine, such as Full Pay Deuces. However, most versions do have a slight house edge in the pay tables. For example, 9/6 Jacks or Better has a casino of about .5% with best strategy.
However, there are sometimes comps and cash-back available with VP. In evaluating the VP game. You have to compare the value of these with the EV you give up on the machine.
I usually figure that in 9/6 Jacks I make mistakes that probably cost another half percent. Therefore I will usually only play if the cashback./comps pay back more than 1%. Surprisingly, there are times when that is the case, especially if you consider the benefits of the mailing list.
Really good VP players obviously make almost no mistakes, and can therefore play a lot more games.