Have you seen your doctor to confirm you are pregnant? If not, please do so. To presume you are pregnant without confirmation may be jumping the gun.
It is possible for a vasectomy to spontaneously regenerate, though it is unlikely and I believe instances where it has naturally occurred are fairly rare.
Have you had any kind of physical relationship with a man since you were with the man who had the vasectomy? If so, and if you are pregnant, it is possible that the new man could be the father. It is not always necessary for the man to ejaculate inside of you in order for you to get pregnant. Unprotected penetration, even without orgasm, can lead to fertilization due to the millions of sperm commonly found in the initial lubrication on the head of his penis.
Are you certain that the test your former partner showed you was genuine? Although it is uncommon, there are men who choose to trick the women they're with into believing they are sterile when they are not. Men who do that have even been known to go on to provide "proof" (which of course, isn't worth the paper it's printed on).
If you are pregnant, I suggest you tell your doctor everything and seek his or her advice regarding paternity testing. You will want to do this as soon as possible so as to take whatever legal steps may be necessary regarding custody, visitation and support.
If you are not pregnant, I suggest you make sure to use birth control of your own from now on rather than relying on the man to be truthful with you. As they say, an ounce of protection is better than a pound of cure.
Good luck!
