Hi Bill, Welcome to the Stamp Community Forum. Vatican City stamps are fun. Lots of very interesting topics.
As to the selvage, its sort of up to you. It's your collection. Do what makes you happy. Assuming the stamp with the salvage is not a highly valuable item and the selvage is blank (no plate #, markings, etc.), there are about 4 alternatives:
1) do nothing and look for another stamp;
2) tear off the selvage;
3) fold the selvage behind the mounted stamp;
4) mount the stamp with the salvage displayed.
Here is SCF discussion on selvage from a few years back:
https://goscf.com/t/18122&SearchTerms=selvageAgain, welcome and enjoy.