Eagle County Regional Airport: Best value: Core Transit’s Valley Route from the airport to Vail Transportation Center. It runs via Eagle, Edwards and Avon; allow about 60–75 minutes and pay about $3 in the Core Transit app (under-18s ride free in the stated season). A shared van is faster door-to-door but roughly $70+ per person; use one only if your flight lands outside the bus timetable or you have a group.
Denver International Airport: Do not rent a car solely for Vail in winter. The sensible budget route is the A Line from DEN to Denver Union Station, then Bustang’s westbound service to Vail Transportation Center; allow roughly 3.5–4.5 hours including the connection and around $25–35 total. If the timetable does not work, book a shared airport shuttle to your hotel: usually around 2.5–3 hours in clear conditions, but I-70 can turn it into 3+ hours on winter and summer weekend afternoons.
Vail Transportation Center: The useful intercity stop is Vail Transportation Center, 241 S Frontage Road E, between Vail Village and Golden Peak. Bustang links it with Denver/Frisco eastbound and Eagle/Glenwood Springs westbound; it is a much better arrival point than trying to drive into the village and immediately hunt for parking.