It's a deal breaker. I left a marriage in part for that same reason. I truly believe that the desire for a child is so deep that you shouldn't give it up for anyone. You'd live to regret it.
He's entitled to not want more children. You're entitled to want them. Unfortunately, it's not something that can be worked out to a compromise.
As far as him having a vas, I see no reason for him to lie about that, especially because it may cause you to go off any contraception you've been using now he's told you. I also suspect that he had it before you got together. It'd be pretty hard to hide the post-op care period (soreness, ice packs, stitches, etc) unless he lied & told you he was out of town or said he had surgery for something else. Either way, that's a pretty shitty thing to lie to someone about. Of course, he could have kept it to himself thinking he'd get a reversal if he changed his mind, but still, he should have told you that.
So tell him that you and he are done. You don't have to make a big scene about lies, expectations, etc. Simply say that you've thought about it and you have decided that you are not going to give up the dream of children and that you and he are through. But please don't let him string you along with 'maybes' and 'somedays' and promises he has no intention of keeping. He has stated it bluntly and (imo) it's very reasonable that a 42 year old man with 3 children wouldn't want to start over. Believe what he has already said, not what he may say to try and keep you.
You've got years left. I had my first just before my 29th birthday. But don't waste any of those years on a man who doesn't want the things you do. Get free, relax, and enjoy your life. Don't rush into single parenthood just now. The right man will come along for you in time. In the meantime, work hard, save money, and find a good area in which to live. Then, years from now if Mr Right hasn't appeared (but I'm sure he will), you'll be better placed for single parenting.