I agree with pretty much everything people have said on this thread. What a great forum this truly is.
I think that in addition to the relationship problems you have, there may well be an ex-smoker problem. People who have quit smoking (and good for them!) can be very hard on those of us who cannot. It was probably extremely difficult for them to quit and as a result there's often a "If I can do it, anyone can" attitude. Their thinking doesn't allow for the variance among people and that some of us have actually become permanently addicted, unable to function reasonably without.
My suggestion is that you proceed in the following order. I should qualify this advice by admitting that I've had two failed marriages and am now married for a third time. Against that admission I want to add that this third marriage has been my longest (going on 20 years) and is a wonderful partnership. We are the best and closest of friends I can imagine.
#1. Get hubby on board. The crucial things for him to understand are that
a) You are rewired to need nicotine. It is not just a matter of willpower.
b) You are taking an enormous step here by switching to a source of nicotine which isn't very harmful to you and isn't at all harmful to others.
c) It still isn't an easy step and you want his support to help you succeed.
d) It also happens to be cheaper than smoking.
To get hubby on board find some posts on these forums which might be most helpful toward understanding all that and get him to read them. Reinforce that you are taking a hard step here and an enormously good one to deal with the problems of your smoking addiction. One thing which might be enormously helpful to convince both him and perhaps later on your mother in law at least of the reduced badness of this is the paper towel demo. Take a puff from a cigarette and blow out through a paper towel. Take a puff from your e-cig and blow out through another towel. Show the two paper towels. That's how much good you are doing with this great effort you're undertaking.
#2. If you can't get hubby's strong support, I'm afraid there is no #2, my suggestion ends here. I'm very sad for you if this happens. You may have to think about what this means in your life.
#3. Tell hubby that his mother is undermining this difficult good thing you are doing and he must address that. Point out her problem with thinking that because she did it she thinks it is the same for everyone and she isn't giving you credit for dealing with the problems you have vs. the problems she had.
Offer to help him in dealing with his mother if he thinks you can, e.g. showing forum posts, blowing through the towel demo, etc.
But keep it his problem. If he's on board that really is what this is. He supports you, his mother doesn't support you, you don't have an inseperable bond with her, he does. So he can tell her she's wrong or out of line, you can't.
If hubby can't deal with the problem after he's on board, again you have my sad sympathy. You'll have to think about what this means in your life.