A Vampire's Bite action is described as:
Bite. (Bat or Vampire Form Only). Melee Weapon Attack: +9 to hit, reach 5 ft., one willing creature, or a creature that is grappled by the vampire, incapacitated, or restrained.
Does this mean a vampire has to succeed an attack roll and the target has to be willing/incapacitated? Or are these requirements mutually exclusive? It seems odd to me that you can miss when biting a willing target.
Assuming it is happening outside of melee, at a minimum. If it is happening outside of melee it is not an attack. The willing victim is offering its neck, and the vampire is biting it.
The counterargument is absurd: The vampire slips in through the bedroom. The occupant says, "master I am here", then the DM says okay, roll initiative. But . . . it's not combat, there's no initiative, and the vampire doesn't have to make an attack roll. Shaking someone's hand isn't unarmed combat. Grappling a willing target outside of combat isn't an attack either, you're putting your arms around them. Putting on a bandage doesn't involve a to-hit. Picking up a kitten bumping up against your leg doesn't involve a grapple, you just pick it up.
If for some reason that doesn't convince you, rulings over rules. If having the vampire bite a willing target outside of melee seems like a nonsensical interpretation of the rules, then make a ruling. From Tasha's:
The rules of D&D cover many of the twists and turns that come up in play, but the possibilities are so vast that the rules can’t cover everything. When you encounter something that the rules don’t cover or if you’re unsure how to interpret a rule, the DM decides how to proceed, aiming for a course that brings the most enjoyment to your whole group.
In combat is a different thing. The PHB says:
A typical combat encounter is a clash between two sides, a flurry of weapon swings, feints, parries, footwork, and spellcasting.
In that case, the DM will need to judge if a to-hit is needed. Perhaps the willing target is being protected by allies. An attack roll makes perfect sense.
Or, perhaps the vampire and the willing target are alone behind cover. The target is willing, and maybe even helping. The DM may judge that a to-hit is not needed, and the vampire simply succeeds.
This is no different from hitting an unconscious or an incapacitated creature, or from hitting an object in combat: in all of these cases, you still have to make an attack roll. There is no explicit rule that you automatically hit stationary targets.1
The PH covers this on page 194, explaining how to make an attack:
Resolve the attack. You make the attack roll. On a hit, you roll damage, unless the particular attack has rules that specify otherwise. Some attacks cause special effects in addition to or instead of damage.
As the text says, the vampire can only bite a victim that is willing, for example because they were charmed by them first, or that is grappled, incapacitated or restrained. For grappling, the vampire has the option to first use its unarmed strike, which can grapple on a hit instead of dealing damage.
It's up to you for how to narratively justify this. Here are two pointers.
First, most combatants are wearing armor to protect their vulnerable body areas. If you are wearing a chain mail coif, a bascinet, or a gorget it may not be so easy to reach the jugular vein. If a target is particularily lightly armored, the DM can grant circumstantial advantage. (Thanks to @ShadowRanger). The DMG also talks about that, on p. 238 where it talks about adjucating attack rolls:
Call for an attack roll when a character tries to hit a creature or an object with an attack, especially when the attack could be foiled by the target's armor or shield or by another object providing cover.
Secondly, combat is not a quiet, static experience, where you have a lot of time to find the vein. Everyone is moving around, including the vampire, everything is happening simulataneously (even if it is broken up into turns for playablity), and the vampire also has to watch out what the other opponents are up to. Here is the PH on it:
In combat, characters and monsters are in constant motion, often using movement and position to gain the upper hand. (p. 190) In a fight, everyone is constantly watching for enemies to drop their guard. (p. 195)
Outside of combat, the DM can just rule that an action succeeds automatically, and that no dice need to be rolled.
1 I do agree this feels a bit weird to me too. To relate a personal story, a friend of mine managed to miss a small suitcase that was lying on the floor with a flail when trying to smash it — outside of combat. Thankfully our floor survived. So you can miss.
The bite attack describes as valid targets "one willing creature, or a creature that is grappled by the vampire, incapacitated, or restrained." A creature who is not any of these things cannot be attacked with a bite. However, merely having a target who is willing is not sufficient to make the attack automatically successful, any more than having a target who is unable to move (such as when restrained or incapacitated). So why not?
The vampire has slipped into a willing victim's bedchamber. The victim presents themselves, and the vampire initiates the draining bite. Either there is no one else present, or those who are present are unable to do anything but watch on in horror - they are held, or too far away, are scrying the scene but not physically present, etc. This is not a conflict, and the vampire should be permitted an automatically successful bite.
The DMG Section on Using Ability Scores (p.237) says:
When a player wants to do something, it's often appropriate to let the attempt succeed without a roll or a reference to the character's ability scores...Only call for a roll if there is a meaningful consequence for failure. When deciding whether to use a roll, ask yourself two questions: Is a task so easy and so free of conflict and stress that there should be no chance of failure?
Here, the vampire has all the time in the world, and no distractions that will prevent them from draining their victim. An attack roll is unnecessary, if this scene is even 'on screen'. It is a similar situation if the victim is unconscious and alone - absent any combatants, we can indeed assume automatic success. As a caveat, though, even if the bite itself is ruled to be automatically successful, the damage from the bite still counts as "harmful to the target" and thus permits the victim a new saving throw against the vampire's charm. They may not be a willing victim for long.
Suppose you have the same vampire and willing victim, but now there are other characters on the scene - in particular, party members who want to save their charmed ally. The vampire can attempt a bite on a willing creature, but this is in the middle of combat:
The clatter of a sword striking against a shield. The terrible rending sound as monstrous claws tear through armor. A brilliant flash of light as a ball of flame blossoms from a wizard’s spell. The sharp tang of blood in the air, cutting through the stench of vile monsters. Roars of fury, shouts of triumph, cries of pain. Combat in D&D can be chaotic, deadly, and thrilling.
The vampire cannot focus its full attention on the victim, willing or not.
In combat, most creatures stay alert for signs of danger all around
In addition to biting the victim, the vampire is simultaneously defending itself against the PCs, perhaps even taking and regenerating damage. It is commanding its minions. It is alert for the presence of anyone trying to sneak up on it. It is evaluating whether or not to use its Legendary Resistance in response to a failed saving throw.
Here, the attack roll does not represent so much the chance that the vampire is able to bite a willing victim, but the chance that it is able to do so uninterrupted from everything else that is happening within the chaos of combat going on around it.
Narratively, you could consider the vampire as leaning in close, scenting the warm blood of its victim, the heat of their living flesh so freely given - and then someone drops a sunburst spell at the last second, interrupting their bite, just when they were so tantalizingly close.
It is the DM's prerogative to assign advantage as a circumstantial bonus - and certainly advantage on the bite's attack roll would be appropriate for a willing victim, just as this advantage is automatic on an unconscious foe.
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