Play it safe - use priority pass and use the VS number. The alternative is to leave the FF number unfilled and retroclaim with Virgin. With some luck you can use your FB plat status for fast track by showing the FB card and saying you forgot to add it to the boarding pass. But if you want the VS miles, forgo this perk it if needed. I would stick to the PP lounge andI would not try a lounge with the FB card since then likely your ticket will be linked to FB.

putting in one number at some point and trying to change it later looks a recipe for problems to me.