My recent observation of the game lead to this article. I've seen quite a dozen different decks or even more in the last week and either played with/against them or just watched the game. I am aware of the fact that such inputs are hard to generalize since meta game is an important factor and an observer playing in another playgroup might get a totally different result. So here are my thoughts:
This card is pure gold. The king of pool gainers. It's not only good in master heavy or big cap decks, but also owns vessel or blood doll in many mid cap based decks. In all the recent games I have never ever seen the drawback of the card causing any problem. Instant pool gain is always strong, remember Minion Tap, but having the ability to play two of them in the same round? Ridiculously strong. It can totally save your ass. Besides there is still the notorious and infamous Villein + Giant's Blood combo, which I have seen many times. Just enough text here, this card is purely awesome.
Overrated in most decks, with some exceptions. If you need the pool fast Villein is way better, but even Blood Doll will generate the pool faster. In case you play a deck in which your vampires might need blood back on them from time to time Blood Doll is just way better. This would be certainly the case for almost any stealth bleed, tap and bleed or weenie deck. The only archetype I can think of which really could profit from Vessel is the wall deck. In a deck were you want to ensure a permanent pool income (via Rack, HG, etc) and also have quite a lot of master cards Vessel is the top pick.
With the appearance of Vessel Blood Doll became barely used. I think this tendency is slowly turning around and Blood Doll might see more play in the future. I described some of it's advantages above. Remember you can prevent a vampire from hunting by just playing a Doll on it and use it right away! It is the faster pool gain and the more versatile card and its drawback you ask? Well in the recent past when I played a Blood Doll it never got removed. Since bleed decks want to play with master canceling cards anyway I suggest playing Wash + Blood Doll. Both cards are very useful on their own, but Wash can protect your Blood Doll. Also Wash is trifle so you can play a Blood Doll on your 0 blood bleeder guy after playing the Wash on your preys Villein...