Firstly, no chemical can tighten the vagina. Vinegar and other chemicals can upset the pH and the friendly bacterial populations in the vagina, leaving it prone to infections. Douching is harmful. Don’t put chemicals in your vagina. OK? OK. Vaginas are made of muscles, and there are some very simple and effective exercises to firm them up so that they don’t feel so loose, and so you can squeeze them to make them feel even tighter. As a bonus, the exercises can improve everything from bladder control to labour to orgasms. Please see the muscles page, or do a search for Kegel exercises. Finally, often when people describe a vagina as tight or loose, they are referring to how easy it is to penetrate that vagina, which is really a function of how slippery it is. Slipperiness prevents chafing and injuries, so I recommend always keeping sex slippery. Muscular squeezing is a better way to increase sensation than dryness. more
^ what this poster said was spot on. Some creams work but vinegar is a bad idea.
here is more info on the exercises: