Vasectomy has a 0.07-0.4% failure rate. The odds of it are slim. That being said however, did your partner ever go in and have follow up tests to make SURE he was actually sterile?
yes he did... and I am stumped, because there are no other possibilities!!
If you've not been with others, I wouldn't worry about it. Even though the odds are very slim, failure always does happen. Nothing is 100% unless you have all your reproductive organs removed. If he doubts the child is his at all, I would have a paternity test done when the child is born.