It is true that a vasectomy reversing itself is rare - but I ran into a little piece of information about that recently. Vasectomies have a 1 in 400 chance of failure/reversal when the man neglects to return for his sperm count tests (the chances are 1 in 400,000 if he gets two zero count tests). Did your husband get his two sperm count tests after his vasectomy?
It could also be something going on with you. Have you been under a lot of stress? Have you gained/lost a significant amount of weight? Your hormone levels could be affected by many things. You should definitely test to see if you are pregnant. If you're not - see you gynecologist so they can find out what's going on. A missed period is a pretty reliable sign that something is going on that needs attention.
Everything does have a failure rate, so it is possible, but the odds are very slim. A period can be delayed or skip for a dozen different reasons or no reason at all. You might not have ovulated this cycle which could cause it as well. Take a test with first morning urine or have blood work done to see if you are or not.