Depending on how one approaches NFP, it is disingenuous to say that there is still the possibility of children at certain times. You’re trying to avoid children, and trying to get something else from sex. That’s the point, that sex between married couples still holds real value when procreation is excluded. It is the manner in which it is excluded (contraception vs preiodic abstinence) that matters in Catholic sexual ethics.
NFP techniques are constantly fine-tuned so that the possibilities of pregnancy are minimised. At certain times in the cycle, they are effectively 0% (eg post ovulation phase). The chance of someone who has had a vasectomy impregnating his wife will be greater than the chance a woman will get pregnant in the post-ovulation phase (assuming ovulation correctly identified).
I’m not sure if you have any protestant friends you could discuss this with, but it may help you to get another perspective. They simply don’t understand things the Catholic way.
As a father of four myself, I can understand that your husband may not want any more children. It is hard work at times and stressful. Responsible parenting may be, in his view, to stop there. Catholic ethics would not prohibit that. It is merely the manner in which he wishes to do it. Catholic ethics do prohibit a vasectomy, but his own ethics do not. So I think it would help to try to understand his viewpoint better.