Vulnerability Effect?

I would suggest that you use "before entity is hurt" procedure external call and make it so that it checks the entity if it has the potion effect "vulnerability" (or whatever you named your potion effect in this case) and if so, check how big the amplifier is, and make it so that it decreases the mob's health per half a heart or more for every 1 amplification amount. So for example if you have "vulnerability" (if you make it decrease 3hp every hurt) and when you are injured, your health will be decreased an extra 3 damage but will not damage you additionally, will only decrease the health so the damage caused by whatever is still the reason you took damage. Here is an example image if this sounds confusing: