When flying squirrel proofing should I seal potential entry points or actual entry points?
Wildlife is persistent about looking for shelter, so if you seal some openings and leave others, animals will eventually find the other openings and use them.  So seal all the openings you can find.

As to the "cement", if the specialist was talking about those plastic caps in particular, then they just meant glue.  "Vinyl" siding is made out of poly-vinyl chloride, or "PVC", and the glue for it is called "PVC cement".  That will only work if the caps are also made from PVC.  If they aren't, then the glue won't stick.  But the reviews say they hold from friction, so you probably don't really need any glue.

Another approach is to fill it in with urethane spray-foam insulation, commonly sold under the "Great Stuff" brand name.  Use the "low expansion" type so it doesn't bulge the trim.