If you are refering to a vow to become a future Buddha, I see no contradiction with Theravada. If you are refering to the bodhisattva vows traditionaly found in Mahayana, take a look at this (the 4th vow) and ponder if you want this kind of commitment:
4. Abandoning the Mahayana.
If you reject the Mahayana, or any part of it, saying that it is not the teaching of the Buddha, you will break this root vow. To some, the Mahayana seems complicated and overly mystical. The teachings assert the existence of countless manifestations of Buddhas and Bodhisattvas. Some people are unable to come to grips with this vast scope and such things as the sophisticated tantric methods contained in the Mahayana. They may come to think, or even say to others, 'The Mahayana is mixed with non-Buddhist practices. It is not a pure teaching of the Buddha as is the Hinayana.' By thinking in this way you abandon the Mahayana and break this vow
He turns his mind away from those phenomena, and having done so, inclines his mind to the property of deathlessness: 'This is peace, this is exquisite — the resolution of all fabrications; the relinquishment of all acquisitions; the ending of craving; dispassion; cessation; Unbinding.'
(Jhana Sutta - Thanissaro Bhikkhu translation)