Can a vasectomy fail?

Ive been pregnant 4 other times, and am good at picking up very early signs of pregnancy. My husband had a vasectomy nearly 20 yrs ago, but his last marriage involved a woman with a hysterectomy . Is it possible that even a few sperm are getting through, and ive become pregnant? What are the chances of this happening? I do know that the vas defrense can re-route itself, but in reality, how likely is it?