Is a Mahayana Buddhist *required* to take a Bodhisattva vow? Is the taking of a Bodhisattva vow a requirement(1) in Mahayana, or is it merely an option (or maybe at most a recommendation)?

And if it's not a requirement, then for someone who decides not to take it, what if anything is the difference, in terms of the overall aim(2), between the Mahayana approach and the Theravada approach?

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(1) By "requirement" I'm invoking the usual idiomatic use of that word -- i.e. pointing to an action and its consequences. For example, "if you want to build muscle, you are required to lift weights" or "if you want to avoid rebirth in a hell realm you are required to refrain from murdering your parents"

(2) Clearly there are differences in practice formats, but I'm thinking more of the end point. In that context, without the vow to continue as a Bodhisattva, Theravada and Mahayana seem like one and the same.
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.