Ok, getting to another question.

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This one stems from the screenplay version:

What is the big deal with the vow? Why not break it?

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And a combo from the novel:

What is with the “I think I might kiss her” obsession from John? What’s the big deal about kissing? Does it mean something else?
(Yeah, put the something else in “”)

To answer them all at once, the big deal with the vow is very cultural. To break it is to lose honor. And, consider the explanation of the vow.  Merrik risks losing power by simply breaking it.

Kissing. That relates back to the vow. In real life, kiss all you want. Just be careful of mono. In the AOM world, if you are a sword master or wizard or other magic user, be careful not to lose your life. You are transferring your power to another, whether or not you want to.

The cautions of the vow become part of the culture, and even non-magic users (who may not know the origin) are cautious of it. As for “something else” it does not refer to that. Think of how much more power a character would lose that way. To answer an unspoken question, there was none of that between Merrik and Brianna. Good thing, Brianna did not need to absorb any magic power, and Merrik did not need to lose any.