Hi!
If you were my patient, I would advise you that it would be quite unlikely for you to be the father. The sperm count should go to zero about 3 months after a vasectomy and then remain there. It sounds like yours has done.
A DNA test never hurts, but I would only do this if she is requesting something like child support--and then you want to establish a good chain of custody for the evidence. (Your attorney can help you with that)
Good luck!
Dr B
Thanks. So are all the flukes on the internet less common than it seems?