Vehicles You remark, [. . .] yanas are incompatible. They are all valid, but you can only use one at a time. That is not the general understanding in at least one of the major schools of Tibetan Buddhism, in which it is regularly claimed that (1) all three vehicles, far from being incompatible, are to be practised simultaneously -- the lesser vehicle outwardly, the greater vehicle inwardly, and the mantra vehicle secretly, and (2) vajrayana differs from mahayana only in its methods (and thus in the time it takes to reach buddhahood, thereby), not in its view. I do not say that those claims are correct -- only that they are believed by a goodly number of Tibetan Buddhists. Whether dzogchen is subsumed under vajrayana, or is a discipline separate from it, is another, and vexed, question.