## Yes, it has to make an attack roll in combat

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.<sup>1</sup>

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. 

### Narrative explanation

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][1], a [bascinet][2], or a [gorget][3] it may not be so easy to reach [the jugular vein][4]. 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. 

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<sup>1</sup> 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. 


  [1]: https://en.wikipedia.org/wiki/Mail_coif
  [2]: https://forgeofsvan.com/product-category/shop/helms/
  [3]: https://en.wikipedia.org/wiki/Gorget#/media/File:Gorget_(PSF).png
  [4]: https://my.clevelandclinic.org/health/body/23148-jugular-vein