Exactly how many months after the vasectomy did you get pregnant?
Did he go back for the required two checkups to make sure he was sterile?
Did he sign the waiver that protects doctors against such lawsuits(the doctors won't operate unless he did, but staff make mistakes).
You might sue and try to get a settlement from the insurance company but know that they play hard ball and will totally investigate your life, including any insurance claims or lawsuits you attempted in the past.
My SIL and BIL ended up in huge legal trouble as a result of many truths coming out this way.They lost custody of their kids, the lost another lawsuit(the jury thought they were repeat scammers), and they had to pay money back to avoid prosecution and being sued.
Now if they ever have a legitimate lawsuit, they have about no chance of winning.