Perhaps I am misinformed, but I was told that digest stacks an acid vulnerability and hits harder over time.

It does not do that (now). Just killed the eye of the deep and digested him... hundreds of times. As well as all the levelling on spirits/sahuagin... and digest never seems to hit harder, even after repeatedly "covering in digestive..."

It doesn't do an acid vuln, it is better than that.

If a given mob has HUGE dam resistance, then a little digesting won't necessarily make you see a huge difference. BUT it still makes a difference. Every bit of damage counts.