I would recommend never choosing a site based on rakeback alone. If a site offers 25% rakeback, but charges 50% more rake, then you're losing out. If a site is soft enough that you have a higher winrate than you would see from increased rakeback, then you're losing out. Rakeback should be considered, of course, but not in isolation.
Your question about VIPGrinders depends. They, and other affiliates, might offer rakeback on a site that doesn't offer any directly. They might offer other incentives, rake races, etc. It really depends on what site you're playing on. As for how you get a deal with an affiliate if they offer one, well, I imagine you'd contact the affiliate.
I don't have any answers for you on best RB deals; hopefully others will chime in. Also, I'd suggest reading the last several pages of this thread.
Good luck!