Is saying, "If I marry her, I must fast two days a week," considered a vow, especially with the intention of deterring the person from thinking about marriage rather than actually marrying her, and without remembering the exact wording of the statement?

If you are not certain that you uttered the vow (nadhr), then it is not binding upon you, because a vow is not contracted except by verbal utterance. What you have mentioned indicates whispers (waswas) and doubt.

Even if we assume you did pronounce it and intended to ward off the idea of marrying the aforementioned man, it is a vow of contention (nadhr lajaj).

The Malikis hold that a vow of contention is binding and obligates the one who made it. However, the majority of scholars believe that the one who breaks it has the choice between fulfilling the vow or performing an expiation for an oath (kaffarat yamin), and this is the relied-upon opinion.