Sadly, sounds kind of typical for the motorcycle service industry (independent). I see the dents in the sides in the pictures, did you do bodywork to remove them? If so, how?
On the one hand I think it's kind of good etiquette to not just flat out accuse them off the bat, but if you had everything tested and fitted before and you got it back from them and it doesn't fit, then it seems quite clear that they did something to it. Is this a radiator type shop which probably does this sort of tank repair a lot? If so, that would at least mean they would pressure test it. Normally of course they would never test it at a PSI that would do that to the tank, but if they had the wrong pressure that at least makes sense.
I was also thinking that since you had an acid (vinegar) in the tank, is it possible that you gave it to them with the lid closed (or they closed it) and then it sat for two weeks and produced gas inside to the point of expanding?
I have done acid treatments and even after rinsing several times, it can still react to putting in gasoline or fresh baking soda water. In other words, they probably got the tank closed, let it sit for 2 weeks (as is), told you it was "soaking" in dawn while it was really just sitting there.
If you didn't pull out the dents yet, do they look different than they did before? (pressure on the inside might change their size)
Seems like you need to speak to these guys and get some answers. Unfortunately, if they did anything stupid they probably won't admit to it.