Vaseline is NOT a good idea.
It is NOT designed for internal use.
If one wants to use a lubricant for tampon use -- or other vaginal purposes -- use a lubricant MADE for internal use, such as Astroglide, Liquid Silk, Wet, KY, etc.
But you don't want to be putting petrolatum in your vagina.
(And FYI? There aren't static states of vaginal "tightness." The vagina tightens up when unaroused, or when a person is feeling nervous, etc. But, for the most part, sparing very extended age and possible issues post childbirth, pretty much all vaginas are at about the same resting "tightness.")