Mylar is inert and won't harm the coins, which PVC has the potential to do.
However. I have noticed that Mylar seems to be affected by humidity and a Mylar / cardboard 2x2 can stick to the inside of the pocket of a PVC album page.
Also, Mylar is not 100% perfect in preventing the deleterious effects of PVC in a PVC album pages blackening red unc bronze coins being affected over a period of years.
For this reason, I suggest that you get rid of ALL of your PVC album pages, and replace them with either museum quality Mylar or polypropylene album pages.
I know; most of my red unc bronze suffered before I made the change. I ended up re housing about 2,500 coins. I needed about 130 new album pages with 2x2 pockets, but the cost was well worth it.
Interestingly, they did not cost me that much more than PVC album pages.