Is joe Biden withholding his  vp pick so she can’t be researched ?

 No, it's the opposite. 

Whoever Biden picks she's going to be thoroughly "researched" by the media and the opposition (in fact many of the potential nominees are probably already "researched" by Trump's campaign).  A few extra days or even a couple of weeks, isn't going to make a difference.

Instead, Biden is delaying the announcement so that he can fully vet his nominee and make sure they're the absolute right pick.

John McCain rushed his VP pick and look what happened there.  Sarah Palin ruined his chances.

we'll find out in nov

He is waiting until the official Democratic convention when he is officially declared the nominee

we'll find out in nov

Don't be silly. There is plenty of time. He can do it during nomination process, which is the many are introduced. If you are too dumb to figure it out it is Micelle Obama or 2nd choice Kamala Harris. There isn't even a 3rd choice.