What are the odds of a women with her "tubes-tied" and a man who had a vasectomy creating a child?

How long after the vasectomy did she get pregnant? My husband is getting a vasectomy after our 4th little babe is born and we just found out that you have to use protection for the first YEAR because it is still possible to get pregnant. Although it's a slim chance, I wouldn't be so quick to ume she cheated. The fact that she got preggo with her tubes tied is kinda crazy too. Guess it was meant to be :)