What happens if a vampire can't rest in his coffin?

Question

The lore text about vampires (MM, p. 295) states (emphasis mine):

Every vampire remains bound to its coffin, crypt, or grave site, where it must rest by day.

How should this be interpreted? Does it simply mean that vampires can only sleep in their coffin (or similar), and if they don't, they just suffer the usual exhaustion rules for not sleeping?

Or are there other consequences? There doesn't seem to be additional information in the 5e Monster Manual, so information from previous versions or general vampire lore is also valid.

(I'm not considering differences between PC and NPCs/monsters. Sure, NPCs usually don't take rests, but that's just for convenience reasons. If the vampire is your BBEG, he might run away and heal up with a short rest before the players catch up to him. So my question disregards whether the vampire is a PC or NPC.)

As a side note, do vampires actually sleep at all? The general undead theme seems to be "no food, drink or sleep", so do vampires "resting" in their coffin simply linger around awake for 8 hours to get a long-rest's benefits?

Answer 1

A vampire that can't return to its resting place becomes a vampiric mist.

As detailed in Mordenkainen's Tome of Foes (which came out just a month after I posted the question) on page 246 (italic emphasis mine):

In billowing clouds of fog lurk vampiric mists, the wretched remnants of vampires that were prevented from finding rest. Indistinguishable from the mists they lurk within, they strike unseen and undetected to bleed their victims dry.

Former Vampires. Vampiric mists, sometimes called crimson mists, are all that remain of vampires who couldn't return to their burial places after being defeated or suffering some mishap. Denied the restorative power of these places, the vampires' bodies dissolve into mist. The transformation strips the intelligence and personality from them until only an unholy, insatiable thirst for blood remains.

I think the focus here is on the second part - "after being defeated or suffering some mishap". If the vampire decides to chill with his vampire buddy next town and therefore skips a day's sleep, I don't think he'll instantly die, just like a human won't die from skipping a night's sleep. However, if adventurers reduce him to 0 HP, triggering the vampire's Misty Escape, and he can't go to his coffin - then he does indeed die (as specified in the Misty Escape feature, except it doesn't mention Vampiric Mists).

Vampires suffer the same consequences from skipping sleep as regular humanoids.

Contrary to the average "Undead Nature", vampires don't ignore their needs for sleep, drink, and food. They only ignore their need for air, as specified in the MM on page 295:

Undead Nature. Neither a vampire nor a vampire spawn requires air.

This means that, since nothing states otherwise, they require sleep as much as your average humanoid (elves aside).

Indeed, the lore text states that they "must rest by day". However, I believe this only means they have to rest in the first place, and they can do so only during the day, not at night (though I would probably ignore the latter part in games I DMed). If vampires couldn't possibly be awake during the day, that would make it pretty goddamn easy for adventurers to kill them.
Vampires are CR13, i.e. a "medium challenge for players of level 13". At this level, the adventurers have access to spells like Teleport or Plane Shift, which means they can port out of the vampire's resting place if they're in a tight spot, and they can scry and port back once they've recovered. If the vampire were necessarily asleep from 8 am to 8 pm (roughly), then that would be way too easy.

Answer 2

Reaching the coffin

As far as I know, there are no further explanations.

A similar line appears on Curse of Strahd

A vampire must rest in its coffin during the day. At night, it can summon wolves and vermin to do its bidding.

From Strahd himself, the only situation he dies if he can't reach his Coffin is when he's already at 0 HP

Misty Escape. When Strahd drops to 0 hit points outside his coffin, he transforms into a cloud of mist (as in the Shapechanger trait) instead of falling unconscious, provided that he isn't in running water or sunlight. lf he can't transform, he is destroyed. While he has 0 hit points in mist form, he can't revert to his vampire form, and he must reach his coffin within 2 hours or be destroyed. Once in his coffin, he reverts to his vampire form. He is then paralyzed until he regains at least l hit point. After l hour in his coffin with 0 hit points, he regains l , hit point.

Other than that, it's not explained what would happen if he couldn't go back to his coffin.

If they are out of sunlight, though, I don't think there should be any harder consequences than them not being able to rest.

About sleeping

It depends on what you call "Sleeping", but, as far as I understood roleplaying Strahd and his vampires, yes, they "sleep" in the sense that they are unconscious/vulnerable for some time. Again, it's not defined, as far as I know, if they can choose to not rest, and if they can, what happens if they don't.

Personally, I think most of the vampires interactions and rulings are up to the DM by now (check this question as another example), which is a shame since they could have been more explored in CoS already.

So, the final answer by now, from my knowledge, is...

It's completely up to the DM.