Looks like it’s been discussed, and isn’t a popular technique. As for me I’m with the “just play them” camp on this one.
Purists...LOL! Strings get sounding that way by the windings getting gunked up one way or another. But while I find Vaseline grossly inferior to regular white Eucerin cream, it's better than rubbing nose and taint grease on your strings.
Pharmacist here. What you’re doing is just accelerating the aging process of the alloys in the string by introducing a highly non-polar (read: oily) substance on the surface. Same as the oils present on the skin of your hands, but the effect of either Vaseline or Eucerin is greater since the oil used on those is much more non-polar than your own. Having worked in a compounding pharmacy, the only thing oilier than petrolatum (petroleum jelly) is lanolin which is derived from animal wool. I actually wash my hands with octanol just to get that off.
Cool...now I know why Eucerin cream works better and quicker than Vaseline or taint grease...it's got both petroleum and lanolin. Thx for the more scientific look at them.
I'll have to admit; I am very tempted to try this with the La Bella Gold Flats on my Epi Jack Casady Bass. I've played them a lot - and they are broken in - judging by the tone - but their outer windings are still... well, not "sticky", exactly. Just that, whatever that outer wrap is (copper? brass?) simply isn't slippery and slick under my always dry fingers. I thought it was just the usual "stickiness" that some flats have, but...no. I've cleaned them and cleaned them, but... no change. So, maybe the Vaseline will do the trick. I'd hate to have to replace them; they do sound great on that bass. Look great, too...
I seem to remember a thread questioning whether people automatically put on new strings when they buy used basses. I'm not a germaphobe but not knowing where those old strings have been or by whom they've been fondled my answer is an unequivocal yes!
I always did it just because I never bought a bass with strings I liked unless it was made for me. I never learned to skeeve stuff like that till maybe 20 years ago. But now I really skeeve it!
I don't always change the strings on a new-to-me-bass (used or actually new), but... most of the time I do. I've acquired...4 basses, IIRC, that I did not change the strings on. But, those basses came to me with flat wounds already on them - so, there was no need to change them... I did take the old flats off my '73 Rick 4001 when I got it; which I regret very much. I've been haunted ever since, by the thought that those strings were the original Maxima Flats that it came from the factory with. I should have at least kept the things, instead of throwing them away... But, every bass I've ever acquired, that came to me with round wounds on it? Yes; those things - new or not - got pulled off that bass tout de suite, and replaced with (mostly) flats, or tapes. It's not so much an "Eww! Other people's nasty DNA!" thing, as it is "Eww! Nasty ol' Rounds!!"...