I hear a lot from players that Blood Doll used to be good but now you should just play Vessel because it's basically the same card except it's a trifle. My response is usually quite a blunt "you're looking at these cards wrong". Sometimes I rant for a little after and sometimes I just leave it at that, but I'd like to share my view with you folks. I'd like to look at what makes these cards different, the benefits and costs of each, why you'd choose one over the other and where some players may be going wrong. Let's start with the thing people tell me the most...
Myth 1: Vessel can save you from an Anarch Revolt effect and Blood Doll can't
False. That's simply just not how maths works.
You can activate it in your unlock phase to take a blood off your Vampire, and that'll pay for the Anarch Revolt, that's very true and it's very handy. So if you're on 1 pool in that scenario, it wouldn't matter how many Blood Dolls you had out, you would die before you got to your master phase. Seems better, right? However, people seem to forget two key factors with Vessel:
It costs 1 pool
You can't use it until the turn after you play it
Here is where the maths comes in. If you had played a Blood Doll instead of a Vessel you wouldn't be on one pool in the first place. You'd have not paid the pool cost, meaning you'd be on at least 2. At least because you could've activated the Blood Doll in your own turn, meaning you could actually be on 3.
Remember this, the activation window on Blood Doll is actually earlier than Vessel as it works in the Master Phase the turn before Vessel's required Unlock. Which brings me swiftly to my next point...
Myth 2: The do the same thing
Again, false. I've even already kind of explained why. Blood Doll and Vessel are both used to manage the blood on a vampire, that's correct, but, Blood Doll is primarily used as a means to recoup the pool invested in a vampire, or to gain pool via hunting, taste of vitae, etc. Vessel absolutely sucks balls at this and I wouldn't look at the card as having that as a primary function.
Let's run the numbers, for the purpose of this exercise let's say I have 10 pool.
Blood Doll - I play the card , I take one bead back, I end the turn on 11.
Vessel - I play the card, I cannot activate it, I end the turn on 9.
If we look at it logistically, it takes 3 turns to gain a pool from Vessel. The first turn we can't move a bead. The second turn we take a blood from our vampire, which pays for the Vessel. Turn 3 we take another blood which becomes the first pool gain, but we've net lost 1 bead for the pleasure. Admittedly, it's all uphill from there, but you're likely already deep in the game at that point.
Myth 3: It's pointless playing Blood Doll because people just blow them up
I know I just mentioned destroying Blood Dolls as an upside, but let's be real, it's not worth much. Okay, so long term you may deny them some pool bloat, and theoretically you've denied them their Master Card at the cost of a Trifle. Is that anything to brag about though? If you're playing a deck that wants to grind, then it's not particularly important where the beads are. Not to mention, they've already gained a pool from playing the card. If I play a Blood Doll, and you blow it up with a Vessel, you've spent a pool, and I've moved a bead back to my pool. Realistically, I'm still ahead on that exchange. The other thing to consider here is whether you're going to hold on to that Vessel. You're my predator in this scenario, which means you play before me. Its likely you'll get your vampire out the same round as me, but you play your master first. Do you hold your Vessel an extra turn to blow up my potential Blood Doll? If you wait, you're limiting your hand flow and slowing your own game with your Vessel denied an extra turn of use. If you don't, you get your card active but lose the chance to destroy my Blood Doll. Personally, I think the downside of the second is nothing compared to the first. Slowing your own game to slow someone else's means you could throw away your chance to get ahead. I think the ability to blow up a Blood Doll is a tertiary concern at best, I tend to shrug when people blow my Blood Dolls rather than sigh as I always feel like I at least got some use out of it.
Now let's talk cross table for a minute. Maybe this should've been Myth 4, but I'm not sure there's much to say. I have heard a few people say "You should always blow up someone else's Blood Doll, regardless of where or what's on the table". Firstly, when a game is as decision heavy and eclectic as V:tES, any advice starting with "You should always" is probably not worth much. If plays were always so linear that the choice could apply in every game, well, it wouldn't be worth playing...we'd all know the solves. The fact is decisions like this require table reads. Personally, when I play a Vessel, destroying a Blood Doll that doesn't belong to my prey means I need to assess the table (like with any play really). If I'm likely to get an oust in this turn or soon, I may blow up a Blood Doll belonging to my grand-prey. However, if my prey looks like they may get the oust first, then I want my grand-prey to have the best chance to survive. After all, that's a tasty VP and 6 pool I could have for myself, if my prey gets it instead it could make them harder to kill. There's obviously more to look for in a game, but that's kinda the biggie.
So, why play Vessel at all? Well, it's not a terrible card, it's just not a Blood Doll. It's handy that you can take blood back with it, this helps us pay for costs, invest in more vampires, keep ourselves alive and so on, it just doesn't do it so well. I find Vessel is much more useful in blood intensive decks where you may need to actively manage your vampires to ensure they can play the cards you need. Just look at the 5e decks that the card appeared in, the Nosferatu and the Tremere.
The Nosferatu deck has 16 cards that cost blood and the Tremere has 22. Both decks have other ways to gain blood, namely Taste of Vitae and Theft of Vitae respectively. Both decks welcome combat, so their vampires need to have at least a couple of blood on them at all times. You can take blood on to the vampire when they're running low, or take excess blood back after slamming a Taste of Vitae for some small incremental pool gain and ensure you get more value from your next Taste. Decks like these you want to have control over where your beads are, and Vessel ain't too shabby at doing that. The Nosferatu deck especially wants the beads to stay on the vampire, where it can take them more easily, so its full text comes in useful with getting rid of preys Blood Dolls.
Also, let's not forget, it IS a trifle, and that's not to be sniffed at! Master phases are important so being able to play Vessel and retain your MPA is pretty sweet. Another interesting thing people often forget about Vessel is that you can play it on other Methuselah's vampires. No, you can't take the blood from it though, the card specifies the vampires controller can move the bead. It's rare and I haven't seen it done, but in a very strange circumstance you could use it to help out another Methuselah in a tight spot. Food for thought.
In summary, Vessel isn't a bad card, but it's no Blood Doll!