Will nailing vinyl over my wall ruin the paint over time?
You need to know how fresh the paint on that wall is and what the sheen of the paint is (matte, satin, semigloss, etc).  Paint that is glossy and less that.. 6 months old will tend to still stick to things that are hung on the wall (I made up that time frame, but feels right to me).  Even matte paint can have this issue, but it's far less likely.

A good option might be to hang up builders paper (basically kraft paper like grocery bags are made of) to provide an intermediate layer between the paper and the vinyl.  The paper can still stick to very fresh paint, but it is less likely to do so.