Is it a GT-R GT-s what motor do you have in it Rb20, RB25 etc.
It could be a few thing the turbo not making boost could be a vacum leak or blown turbo it self if your getting oil in the intercooler then def a blown oil seal in the turbo.
What diff did you put in because if they put in the wrong replacement that could be part of the problem.
Are the vibraitions in the stearing wheel? Whole car?
I'm no expert but I'll do what I can to help you out just need a few more details.