Vasectomies have a greater than zero failure rate. Sometimes the sperm tubes (don't know the name, ain't gonna look it up) grow back together.
If you have a vasectomy, you're supposed to occasionally go get some labwork done to ensure you don't have any swimmers.
They are also hard to undue. Knew a guy who lost his wife, remarried, but when his second wife complained about not having a child, he had the surgery to undo the first and gave her a girl at age 70.
There were two guys on my dept floor who had vasectomies and had to back for follow up surgery because they still had swimmers when they did the initial post op sample checks.
Well then that makes a vas deferens.