It sounds to me that you've thought this out in a very good way. Waiting is a reasonable request, imo. A vasectomy is final just as a miscarriage is final is many ways. I agree that this may be his way of dealing with his pain over the loss of the pregnancy. I suggest that you try talking to him about how you feel which will include talking about the pain of your loss. I suggest that both of you need to mourn that loss before making a major decision about another pregnancy. And mourning usually takes at least a year.
On a practical note, he can't legally get a vasectomy without your signature. Do not sign until you're comfortable with the decision. Yes, this will create more tension between the two of you which you'll need to work out. But if you sign to prevent the tension then you'll still have your bad feelings about the vasectomy.
Hold out until you're ready to have it done. I agree that if he has the vasectomy now while you're still wanting another child you will have resentment and it will negatively affect your relationship. Deal with the feelings before anything is final.
A vasectomy is final. Waiting still allows for a child but it also allows for no child and a vasectomy later.
Later: I was wrong about him not being able to get a vasectomy without your signature. Apparently he can. I would be terrible upset if I were in your place and my husband did it without my agreement.