I'm not sure if such a vow can be made. I say that because either spouse has the absolute right to demand the marital debt at any time, and to refuse the marital debt is sinful. While spouses can mutually agree to periods of abstinence, either spouse can break that agreement and demand the debt, and the other cannot refuse under pain of sin. It would seem, to take a vow of continence before God, may place a spouse in an untenable position if the other demands the debt later (which would be his or her absolute right to do).