DeOxit should never be used on anything except metal, and it should be removed with a solvent before contact conditioner is applied. The boss complained that it did not clean the volume pot on his stereo. I opened it up and R5 had gathered like butter between the front panel and the chassis!
Vaseline is petroleum based, so I do not know if it is a good match with plastic. If you are talking about cheesy sliding carbon faders, where you need to lube the plastic slider, I like a lube I got from MCM that is made for the moving parts in VCRs and the like. It is Luberex made by GC electronics.