Will the elemental dmg sc's i saved up stack with the elemental dmg that vengence does? or are they useless?

It will simple add that elemental damage to every swing you do. Elemental damage from your charms will not be amplified by Vengeance. It will get added AFTER the basic Vengeance damage calculation.

would they be worth running i have 40x of them or should i run Pcombs?

Well avengers kinda suck so be sure to try the build out before investing heavily. Investing a lot into the build and then finding out hitting 1 monster at a time with 1 attack gets old real fast is not fun
Your question depends on too many different factors to be answered like that, create your character in maxroll.gg and look at the damage calculations.
If you have the charms anyway you might as well use them, won't cost you anything.