If he is willing to reverse the vasectomy just to please you and not because HE is 100 percent really, truly on board and craving a baby as much as you are -- please do not do it. Both partners must want the baby. It sound as if he is agreeing to this just to get along, not because he wants a baby. And is he willing and able to care for that baby, without feeling any resentment, if you are laid low after the birth by health problems?
Can you sit down and be really tough with yourself and ask: Am I just craving the baby stage, when they are totally dependent on me? When they are darling, soft, precious little bundles who are so sweet and never talk back, never complain, are just totally in need of me? Am I possibly feeling a bit of grief over the end of those days now that I have kids who are past that dependent infant phase--kids who do indeed talk back at times, who don't need me every moment, kids who are very gradually moving away from me?
Please don't mistake normal grief for losing that baby stage as a need for a new baby. If you follow that path, you'd end up with infant after infant. I've seen families where, once the older kid was starting preschool, the mom started to want a new infant to cuddle and carry, and the results were strained marriages where mom was exhausted and dad was resentful that the family couldn't move on to enjoy the school years, while the older kids were resentful that their activities were always curtailed by "the baby can't do it so you can't go either" and so on.
Explore spending more time with your kids and volunteering at their schools and activities. It is wonderful to be very active in your child's school, preschool, Girl or Boy Scout troop, sport, dance, whatever. That is part of the reward of having a child who is getting older, but it does not happen if you have a new infant.
You have two healthy kids and you came through it healthy yourself -- you are risking your health if you have another child, as you admit. If another pregnancy harms you, you are shortchanging the two children you DO have, leaving them with a mom who may not be fully able to engage with them, help them, be there for them.
Please see if you can sit down and get past your gut emotions that want a sweet infant, and think about what would be best in the long run for the children you already have, and for your marriage.