Is a 4 wheel drive vehicle necessary for Vail in mid-November? Assuming average weather? I'm guessing that could really increase the cost of a rental.

No. It shouldn't be necessary. Front-wheel drive should be fine if it snows and the roads are icy. Unless you're planning on actually doing some 4-wheeling.

We've flown to Denver and then driven to Vail in December and January without 4WD; didn't have any problms.

Thanks for the speedy answers! A friend had suggested that we would, but I didn't think so. Obviously, when the time comes, we'll watch the weather reports.

I live in Boulder and have driven up skiing hundreds of time to Vail and other places. We have never had a four wheel drive vehicle or even snow tires. Just a front-wheel drive with all-season tires. Never had a problem. It is a good idea to have a set of chains with you though just in case. I always carry them but have never had to use them on ski trip. I don't know if rental companies can include them or not. But in any case, don't waste your money on an SUV.