Vaseline is one of the last things I would use for lube on the inner thighs. It is messy, hard to clean off clothes and being heavily oil based, it blocks the skin from breathing. I don't have the problem of skin chaff like a lot of people do but there some much better products out there such as Body Glide and many people also use Mennen Speed Stick Deodorant which works just as effectively and costs 1/3 the price of Body Glide.

Also you may want to look into form fitting running shorts. These are synthetic based which helps to wick away moisture and are long enough (8 inch inseam) that they will prevent thigh chaffing. I have a couple pair of these that I wear underneath my regular shorts while multi-day hiking and I cannot say enough about how well they work. Usually at the end of hiking day, a quick rinse with some soap and water, the shorts can be wrung out and they will be dry within a very short time...can't say the same about cotton underwear.