K.U.
It's easy for him to want more kids but you are the one placing yourself at risk with your pregnancies. Ask him how he would feel if he had the vasectomy reversed, you became pregnant, and then you died because of some complication. If you die, your kids are left without their mother and your husband is left without his wife and needs to raise the kids alone. You also need to ask yourselves if you can truly afford more kids, since every child costs money - diapers, braces, driver's ed, college, etc.
I'm a little biased here, but DH had 2 kids already from his first marriage and we had our daughter 3 years ago. After she was born I developed deep vein thrombosis and ended up with blood clots in my lungs. Fortunately I came through okay but getting pregnant again would not be in our best interest for just that reason. I don't use hormonal methods of birth control for just that reason. So we are happy with where we are.
If you both truly want more kids, then your hubby should still be giving adoption some more consideration rather than placing your health and life at risk.