Yes, a very small risk. BUT it happened to my friend, and was def not the result of an affair as she and her partner had a paternity test when they were involved in legal proceedings. Have my fingers crossed for the right result for you.
My sister knows someone who got pg after the snip. He had been tested and had a sperm count of 5. This is considered infertile and thus the snip had worked. One of those 5 little buggers still got through.
So sadly though it is HIGHLY unlikely you'll get pg again after a succesful vasectomy, it's not impossible.