What's the rule when you need some help installing rubber or poly items? Ok to use a little vaseline whenever needed?

Actually, there are better choices depending on the rubber or Poly part, etc. Here are several:

. Liquid Dish Soap and Water Solution,
. Talcum Powder/Tire-Talc,
. Dielectric Grease (silicone grease),
. O-Ring Lubricant,
. Liquid Rubber Lubricants (like Ru-Glyde).

Another Example, the Rubber Base Gasket on the Oil Filter for my airplane engine, the manufacturer recommends Dow Corning #4, Electrical Insulating Compound (silicone grease).

Vaseline Petroleum (petroleum based).Jelly remains after application, attracts dirt and may even cause deterioration of rubber parts. One reason Petroleum Jelly is not recommended for use with condoms.

My "auto" trans instructor decades ago said vaseline was to be used appropriately in auto trans rebuilding as it is compatible with AT fluid (maybe it just melts easy). He talked about "door ease" the grease stick for the thin black seals that go around the perimeter of the round alum "pistons". He did have a specific area(s) for the vaseline useage I just dont remember off hand where inside it was that he mentioned

I used it on my windshield gasket (truck) and it got hot and oozed out for a year or so....

Quote "One reason Petroleum Jelly is not recommended for use with condoms."

WHAT!!! No wonder I have 4 kids. 8)

You and I could of had the same instructor.
I took a course in transmissions, back in 1980, when i got out of the Air Force.
My teacher taught us to use the same "stuff" when rebuilding our automatics.
Some taught knowledge that probably just isn't passed along to a new generation anymore.