Is it permissible to expiate some vows that exceed one's capacity, in the event that several matters were vowed at the time of marriage?

A vow does not bring about anything that Allah has not decreed, and conditioning it on a specific matter is not from the Sunnah. However, whoever vows an act of obedience must fulfill it. If you doubt the vow, you are not obliged to fulfill it. As for what you have vowed, the default is to fulfill it, but if you are unable to fulfill any part of it, you must offer an expiation for an oath.