Is it obligatory to fulfill a vow to marry a second wife, or is expiation permissible for it?

A vow of obedience must be fulfilled, while in a vow concerning a permissible act, the one who made the vow has the choice between fulfilling it or offering expiation. Marriage is a permissible act, and it is not made obligatory by a vow, even if it is recommended.

Therefore, marrying a second wife is not made obligatory by a vow. If you believe it is better for you to limit yourself to one wife, or you fear you might not be able to treat them justly, or your financial means are limited, then there is no blame upon you for limiting yourself to one, and you must offer the expiation for breaking an oath.