Just got back from Park City two weeks ago. Stayed in North Tahoe last year. Never been to Vail. Where we stayed in Tahoe had pretty much no nightlife as far as I could tell. The admin is right- the South Shore near Heavenly was hopping. Also, the guy complaining about Heavenly- unless you need an endless variety of cliffs I wouldn't listen to him. I thought it was fun. Park City had ONE bar that fit my description of a cool bar- it's called No Name and was pretty awesome. We had dinner at the distillery (High West) but like most things in Park City it's designed to appeal to rich people.
My idea of good night life is trying to find where locals go. No Name seemed to be that place in Park City. I'll tell you something else though that will tell you something about how I feel about local nightlife hangouts- some of my favorite times partying ever at a mountain town was in Glacier (near Mt. Baker, Washington.) We went to a bar called Chair 9 that was packed with people, many of them still in their gear at 8 at night and met so many people and talked about the mountain and boarding and all kinds of stuff. Very friendly and very fun. Plus they have a 30 foot movie screen they play ski porn on the entire time. it was awesome!