Is a Mahayana Buddhist *required* to take a Bodhisattva vow?

One thing that is very particular in Buddhist, is its approach regarding what should one do. In Buddhism, we don't say: You must do A, B or C, instead we say: If you do A the consequences will be X.

Take the preceps for example, you should "refraing from killing", this is different from: You shall not kill. The difference may be subtle, but is there.

So, to answer your question I think you don't need to take any vows in order to practice the Dhamma, the bodhisattva vow is something that you do if you wish to become a Buddha (most likely a private Buddha) in a future life, if you seek liberation, if you wish to become an arahant, there is no need for such vows.

Some traditions, schools, teachers, could incite you to take bodhisattva vow(s) as sort of basic thing. But this is their particular teaching and not general Mahayana requirement. Because Mahayana suppose diversity of teaching for different types of personalities. Mahayana, I repeat, is not some unified school, but very very broad notion including any and all types of Buddha's teaching for any types of beings. Some personality may not have courage to have bodhicitta intent and of course she/he will not be pushed out from vehicle of maha-yana.

Cultivating bodhicitta (both ultimate and relative) is absolutely indispensable on the Mahayana path; and one who has vowed to do so is called a bodhisattva. From my understanding, this vow can be made with as little or as much pomp and circumstance as befits one's mind - the key is to inspire confidence and commitment. Check out Shantideva's 'Way of the Bodhisattva' for a truly awesome and beautiful way to undertake this path. Also see Patrul Rinpoche's 'Words of My Perfect Teacher' and his discussion of the vow of bodicitta starting at p. 220. He says:

"True absolute bodhicitta is attained by the power of meditation and does not depend on rituals. To generate relative bodhicitta, however, as beginners we need some procedure to follow, a ritual through which we can take the vow in the presence of a spiritual teacher. We then need to constantly renew that vow, in the same way, over and over again, so that the bodhicitta we have aroused does not decline but becomes more and more powerful."

The "presence of a spiritual teacher" can be either physical or visualized, based on the various presentations I've encountered.

Based on commitment to bodhichitta, one trains in the precepts of aspiration and application. The final fruit is perfect Buddhahood expressed in perfect wisdom and effortless compassion toward all sentient beings - nothing at all limited or 'private' about it!

As per my understanding, formal taking of Bodhisattva vow (with ceremony and all) is not a hard requirement, as long as one eventually internalizes the core message of the vow: that one must surrender the hope of ever attaining Nirvana and get very comfortable with the idea of staying in Samsara for a long, long time.

Here is a version of the vow we chanted, after every meditation session:

Sentient beings are numberless; we vow to save them.
Desires are inexhaustible; we vow to end them.
The Dharmas are boundless; we vow to master them.
The Buddha's Way is incomparable; we vow to attain it.

See those adjectives -- numberless, inexhaustible and so on? Reciting them again and again makes you question what you're really up to ;)

Of course this trick only works in conjunction with perfectly realizing the rest of six paramitas. Staying in Samsara is an act of murdering the ego, not of easing into worldly pleasures. Murdering the ego is the main prerequisite for Enlightenment, from Mahayana perspective.