If I was a user of this service, the way I'd probably want it to work is:
I always use the non-VIP search by default.
On the search results page, I see all the free results, plus some indication of how many better results I might have seen had I paid.
When I've paid for N searches, don't make me use VIP search next time. The free results might still be good enough for any given search.
One way to do (2) might be a single message on the screen saying "12 more results available with VIP search - see them now for just $0.99!" or whatever. (Or if you'd already paid: "you have N VIP credits remaining, use one now?" or whatever.)
Better still might be to show a placeholder for each VIP search result in-place among the free results. Then I also get to see how the VIP results rank compared to the free results, and perhaps (for example) which page or website they come from, but not any actual matching text. Clicking a placeholder result would take me to the payment screen, or use up one of my credits if I'd already paid.
Of course, this may not be the best business model, as it allows users to do non-VIP searches as often as they'd like to check for "good enough" results, even after they've paid for a number of VIP searches.