It scales to your health?
That's the only one I can think of.
It really gets awful when you're doing the later chalice dungeons. Fortunately I had a stockpile of the blood echo items because I never used them, so I've been chomping those to buy vials. I've ran out of them now though. So if the depth 5 level 2 pthu boss is as hard as people say, I'm going to be farming vials again today. Sigh.
My method of late game vial farming is to kill the three or four
frenzy brain
ightmare Mensis
window
A good run for me I'll typically use two vials for 3 kills. Kill the the first, recovery from status. Kill second and third with a tiny bit of health left then use the escape item. I suspect some of the later Chalices have more efficient ways though.