Should the suitor be informed of a vow to fast on Mondays and Thursdays before marriage, or can this be concealed until after marriage?

A vow to fast is valid and must be fulfilled. There is no need to inform the suitor about it before the marriage contract, as he is a stranger. You can inform him after the marriage. He does not have the right to prevent his wife from observing this vowed fast, because it is like the fast of Ramadan (definition) in its obligation and in not requiring the husband's permission. This applies to every obligatory fast that is time-bound, such as vowing to fast specific days.