I'm still classed as 'vulnerable' too, even though the chemotherapy that got me put onto that list ended several months ago. So my system is also doing its best to recover from cancer.
With the exception of the very small minority of people who might suffer an allergic reaction to its components, an mRNA vaccine can't harm anyone. (There might be a few people for whom the vaccine is unsuitable simply because it wouldn't work for them but it still wouldn't harm them to have it). I'll certainly be taking up the opportunity to have the vaccine, as soon as it's made available to me.
As I see it, your husband's reasoning is totally illogical. He's rejecting something that can't harm him, while leaving himself vulnerable to something that most definitely could!
[BTW: The NHS has been recommending that
everyone should consider taking Vitamin D supplements during the autumn and winter months for many years now. I certainly do:
https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d ]