On what are you basing that number?
I can only speak for myself, but I haven't bought any PS Vita games because they've all been available on PS+. I'm sure there are many more like me.
I think in some cases, PS+ actually makes consumers LESS attached to the Vita, hurting sales in the short and long term.
PSN+ didn't hurt the PS3! So why would it hurt the Vita? At the end they will get the money from your subscription, even if you got PSN+ for free, theres a lot of people who didn't!.
It's very possible it's a profitable system for Sony, but I'm a little skeptical. I think it's a dangerous game giving software away for free when software sales are so low.
I'm glad Sony is doing it -- it's a great service for the fans -- but I think it might hurt overall software sales for Vita, thus discouraging developers from making more games.
I'm sure there are a lot of people who buy games on PS+ they wouldn't have normally because of the price drops, but I'm equally sure there are many who are using the service to save lots of money that would normally go to full game purchases.
Someone should do some research and find out if it's profitable, and whether it's helping or hurting Vita software sales.