I'm glad Vin is back in, he definitely makes the flick better.
If I remember correctly, he had some conflicts with other movies he was working on, so he wasn't in a few.
The conflict was Vin thinking he was too big of a star for a bunch of cheesy Fast and Furious sequels. He has since learned he needs to take the work wherever he can get it.
Pitch Black I think was the conflict or money as you suggest.