Is it okay to have a vasectomy or your tubes tied in order to prevent pregnancy? I would say that it depends on the motives and circumstances. If the purpose of having a vasectomy or tubes tied is in order to be able to fornicate without the fear of pregnancy, then that would be sinful and wrong. Also, if the purpose was to defy God in any way, that would also be wrong.
However, what if the wife has a sickness or genetic difficulty where a pregnancy would kill her? At this point, it would be prudent for the husband to be "fixed" in order to save the life of the wife. I know that this a possibility because this is exactly the case with my wife and I. She has Marfan Syndrome and her last pregnancy was very hard on her heart. We had to go to the emergency room more than once. Therefore, it was prudent, after having had several children, to have a vasectomy in order to safeguard my wife's life. Likewise, I think it is perfectly acceptable for a wife to have her tubes tied if it means saving her life.
What about economics? Let's say that a couple is in extreme financial difficulty and having a child would make it even worse? Should they then take preventative measures? This is a more difficult issue since the motives of the couple and the fact that God providentially cares for people must be taken into account. The motives could not be wrong, of course, and we must acknowledge that God can and does provide for children, particularly when it is a Christian couple depending on the Lord. Besides, waiting for the right economic situation might mean never having children.
What about famine? Another example would be the condition of a famine where there is no food for an additional child. It would be proper at that time to practice birth control/vasectomy.
The point here is that having a vasectomy is not automatically wrong or right. The motives and circumstances must be taken into account. Basically, it would be good to do it to protect a woman's life. It would be wrong if it was for self-convenience.