Vials always recover certain percent of your health. You should be able to heal fully from 2 vials throughout the game. Problem with Vials is that everyone uses them like Estus even though they don't work like Estus. You are not supposed to use 20 Vials everytime you try a boss since they don't replenish like Estus. That being said I would prefer Vials to be like Estus.
The amount of HP blood vials heal scale with your full HP, they always heal the same percentage wither you're level 1 or 100!
Hmm I didn't actually know that. On all my attempts at Ludwig it took 3 vials to top myself off after near death, but I usually would do 2 just to be conservative. I guess it's just more a case of later bosses hitting you harder, so your relative vial use goes up as the game goes on. Well, unless you're stacking vitality to the ceiling that is....
Either way, there really is no alternative to healing on a boss that you haven't mastered yet. I know they're not supposed to be like Estus, but if you get injured there aren't any other options (well on regular mobs you can use rally / heals from visceral attack rune, which I do). On bosses, trying to heal via rally is usually the worse option since it can get you hit again, often fatally.
(Anyway I've complained about the blood vials enough. It's a momentary inconvenience when you run out, but it's still annoying. The rest of the game is so fucking great that it's not a major dealbreaker or anything.)