Partner wants a vasectomy as he doesn't want anymore children but he is willing to consider adoption. Normal?

No, it's not normal. Nor is it "admirable" as some have said, because, let's face it...he's just saying these things because he wants his vasectomy. He figures you aren't keen to adoption, so apease you for now and tell you he will adopt later and in the end, you won't want to. That way, he gets away from ever having any more children. He is so stuck on getting his way now, that he'd say almost anything. It's disingenuous. I wouldn't believe him if I were you. I think you know it too. The weird thing is...he might actually be up for more children in a few years time. Men say women are changeable. Personally, I feel men are extremely short sighted. They only see the here and now, and right now he's overwhelmed with your twins.