You are raising philosophical question here!
I will answered, and not mentionned that one is your parent’s case.
First, in the first case, you mentionned that the “priest allowed the vasectomy”.
If we folowed the Catholic Church (expressed in constant teaching and the catechism), no situation allowed a sterilisation/contraception.
So what? one priest allowed it. Can we followed? yes, of course, I have a free will.
But it is legalism if I do it because of this permission: someone of autority over me, has allowed me, so I can, and relieve my conscience (even If I know that it is wrong). So to be honest, and it’s hard, I know, we will have to accept I do it because I want to, and not because the Church allowed me.
Priests can failed, and some (and they are not exceptions are not opposed on contraception in all cases).
On the second case, I will just add my personal opinion. it is not good to try to adopt after a sterilisation in our current society.
Why? Because there are very few children that are adoptable, and hundred of candidates for a single heathy baby. So if they adopt, it will be at the expense of a couple that suffer sterility. The only way to make some difference in this situation, for a family of 4 children is to try to adopt children that will have difficulties to find parents: handicapped, oldest children, siblings, who suffer trauma…