We have been using NFP and are blessed with two children. Both pregnancies were very difficult for my wife. Both children were breech and the pregnancies were considered high risk. Scheduled C-sections were done because no way that baby would come out feet first. The doctor has told us that we should not have any more because they will likely be breech. They also found an inverted cervix which makes natural childbirth almost impossible and extremely dangerous to mother and baby. The doctor does not want to tie tubes because it will mess with hormones, etc. It was suggested that I get a vasectomy.
From the perspective of Catholic morality, neither you nor your wife can be directly sterilized for any reason whatsoever, even in difficult circumstances such as this. Even from a purely secular perspective, a vasectomy can cause as many risks to your future health as a tubal ligation can cause for your wife. DontFixIt.org is one secular site that provides information on that. You may also wish to consult with the Catholic apostolate One More Soul.
When medical reasons indicate that future pregnancies should be avoided, the only licit options are strict NFP or complete abstinence from marital relations until the end of fertility. For more information on how to practice NFP in the strictest fashion possible to increase its effectiveness in preventing pregnancy, I recommend consulting with the Couple to Couple League.
That Celibate Bachelor Was Right! by Rachel Fay