Is Tibetan Buddhism considered part of Mahayana or Vajrayana?

I've been studying and practicing to one degree or another since high school (I've been out of college two years now) and I've seen Tibetan buddhism labeled as both at one time or another, so which is it? I'm a bit confused.

It is specifically Vajrayana, which is more generally an offshoot of Mahayana. Of course, heated differences of opinion will come up in discussing these sorts of things, depending on one's background, location, and personal orthodoxies.

On one level it could be said that Mahayana teaching reached Tibet, etc., encountered the local Bon shamanism, and became Vajrayana ... much like one might say that when Mahayana teaching reached China it encountered Taoism and became Chán/Zen.

To add to what BTRIPP said: my understanding is that Vajrayana is a subset of Mahayana, as it takes the bodhisattva ideal. But Vajrayana was formed not only by contact with Tibetan Bon, but included Tantric practices even when it came from India, practices that had been borrowed from Hindu tantra.

Most Tibetan Buddhists practice one or another form of the Vajrayana. The Vajrayana by and large adheres to the Mahayana scriptures and practices, but adds another set of scriptures and practices known as the Tantras. Thus Vajrayanists are Mahayanists, but not all Mahayanists are Vajrayanists. The Vajrayana developed in India, and was prevalent there at the time Buddhism arrived in Tibet. Outside of Tibet Vajrayana was widely practiced in China and Japan, and there are many Vajrayanists in those countries today, following non-Tibetan traditions. Mongolian Buddhism is mostly Vajrayanist, but in this case developed from Tibetan sources. Western Vajrayana too has mostly Tibetan roots.

my understanding is similar to that expressed above esp. 4 additionally, I believe that even the 'lowest' level empowerments require the Bodhisattva vows. The tantras can be considered a means of *practicing* Bodhisattva sutra practices.. from one view then they are not different.