I think this may be the wrong forum for this question. This is the infertility forum, mostly women and we wouldn't know about the pain and most of our husbands haven't had a vasectomy. You may want to try the "ask a doctor" forum. Or try one of the mens health forums.
You may get lucky and get an answer though, but you're better off in one of the forums I mentioned above.
This is so rare that it barely shows up on the radar. If it happens though, its normally from a spermocoloe , and all they have to do is re-snip the cords. in very rare cases they denerve the cord under a microscope.