Well the way it reads is that it interacts with the "running style" slider bar when it is set at least 50% elusive. Mind you; I only have 6 points invested in it so far. But, I do believe I can see a difference in the amount of missed tackles my return CB has noticed from last season.
I think considering that a WR or HB on returns has the spin, head fake and juke SA's; the slippery VA can only help a CB who has only the Return Man SA.
A CB can acquire all of the other SA's from AEQ; but it will take all of his career trying to save up the bonus tokens to equip them. The Slippery VA reads a 3% chance each level of breaking tackles.
Match that with a +3 agility with a 4% break tackle chance AEQ piece and you have a grand total of a whopping 70% chance of breaking a tackle on a return if that's 6 upgrades on the AEQ piece.
Add to that the bonus of the return man SA which reads a bonus to the speed, agility and "tackle avoidance" during returns and you have a bada$$ mofo returning kicks.