If you are already intimate with her, sexually speaking, I find it so hard to believe that neither of you discussed birth control. So, if she is on the pill or you are using some other form of birth control, then you are in a very very deep lie.
I also find it so hard to believe that you would propose to a girl with whom you clearly know WANTS children. So, are you faking the fact that you do not want kids on top of keeping your vasectomy a secret? How could two people in love, who are engaged to be married not be honest about children?
I am perplexed.
In any event, you are perpetrating a big fat lie. And, you had better come clean. Marrying her while she actually believes she will have a family with you has got to be one of the top cruel things to do to a woman. How can you betray her that way? If you married me on a lie, I would leave you and have the marriage annulled due to FRAUD. You are planning this wedding under false pretenses.
I have no clue how you sleep at night.
I suggest you tell her RIGHT NOW that you never imagined your relationship would lead to marriage, so you kept your vasectomy a secret which you can no longer keep. Tell her you will understand if she leaves you for a future with a man who is honest about wanting a family.
This is unconscionable.