'My problem is he's got me either way on this. If I tell him not to go through with it he will hold over me the whole"I offered, I was trying for you and you didn't want to make things work". He wins on this whichever way. I can just imagine him telling people "I even did that but she still left me ..." conveniently forgetting the years of crap I have put up with.'
Does it matter?
'I was trying for you...' - answer: 'Well it was too little too late, and only done in order to stop me leaving. Which isn't 'trying', it's 'emotional blackmail' - I don't love you anymore after all the crap I've put up with, so I'm leaving, and your vasectomy, or lack of it, is now your business.'
"I even did that but she still left me ..." - answer: 'I left him because I didn't love him after all the crap I put up with, which included refusing to consider a vasectomy for years despite my medical history. So that's the real story MIL/mum/Suzie down the road, want another cuppa?'
If you don't love him, leave him. It sounds like your marriage is over, so do what's right for you. But don't let any decision you make be swayed by any sort of emotional blackmail - it won't work, it won't mend things. No it isn't great news for your marriage - he doesn't want to 'work on it', he wants to stop you leaving him. And if he's the type to blackmail and twist things in this way, he'll find a way to do it no matter what the topic!
So you have to ignore that and make a careful, considered decision on what YOU want to do. Which sounds as if it should involve leaving him, fwiw - he sounds like an absolute knob. Next time he brings up the vasectomy, keep a dead straight face and say, 'Well, I'm not sure I should have any say in this decision any more. It's such a pity your second wife isn't around yet, it really should be something you should talk through with her. You've talked about her so much she's almost a reality to me now! Sorry, I'm a bit distracted at the moment as I'm so pleased to no longer have to sleep with a selfish twat who regularly puts my health at risk.'