this subject always reminds me of this joke:
a man was caught on top of his car in a flash flood. he cried out to God to save him, beging God for his very life. a man in a little tinny(boat) came up to the car and offered to save the man but he refused to get in the boat saying "I don't need your help, God will save me!"
five minutes later a larger boat came along, threw out a life ring and told the man to grab on. again he replied, "I don't need your help! God will save me!"
a short while later a helicopter flew overhead and dropped a harness down for the man to get into. once again the man cried out, "i don't need your help! My God will surely save me!"
eventually the man was swept away and drowned. when he got to heaven he asked God, "i called out to you for help, why did you not save me?" God replied, "i sent two boats and a helicopter, what else did you expect me to do?!?"
(sorry if this offends, i am in no way trying to limit what God can do. please take this as what it is - A JOKE!)
i don't know if this makes sense to anyone else. but to me it kinda equates to asking God, "why did you keep giving us kids?" i'm sure His answer will be, "because you kept having unprotected sex!"
yes, God has a hand in everything that we do but are we forgetting about free will? our physical actions have consequences, whether they are God ordained or not.
i don't feel in any way that geting a vasectomy or using any other form of contraception is a sin (as long as it is pre-fertilization... but that's a whole other can of worms!).
i think that it is a responsible and wise man who can recognise that he cannot care for any more kids and so takes measures to prevent a child to be brought into an environment where it will not receive everything it needs in ample amounts. like someone said, if God really wants you to have another child then your vasectomy won't stop Him anyway!
good luck with the twins!