I think the vampires were allies of the demons in the holy war. Just Zeldris was kind of a dick and sealed them anyway just to show off
I can buy them being allies given the treaty but them getting sealed because zeldris is an asshole is a bit much. The vampires so far don't seem even more powerful than the sins... If the vampires were enemies it kinda seems like a couple of demons would have been able to easily defeat them. With that in mind it seems kinda unlikely they actually needed to seal them. But then they did get sealed so the issue would be what the true purpose of sealing them was. Its not like the vampires would be of use to anyone when sealed as far as we know... The only reason I could think of that would cause the demons to fear vampires is them consuming demon blood and getting their power. But other than that it does not seem like vampires are strong enough to even match the sins, let alone the demon clan.
So... the vampire clan is not actually a threat to the demon clan but yet they got sealed. So the issue is why they got sealed... Even if there was a risk of the vampires consuming demon blood it is weird that they sealed them rather than exterminate them. With the apparent difference in power it would seem like exterminating them would even be easier. With that in mind perhaps we can make the case that the focus is not in itself the vampire clan but the seal. Perhaps they were testing the seal? The ones the demons seem to really fear are the goddess clan people, they seem to be the ones with the strongest supernatural power. So the vampires were a test since they seem to be adequately powerful, so far it seems like the royalty ones could be on the level with GHKs. So they devised the seal and tested it on the adequately powerful vampires. It'd be interesting if this was the very seal that eventually trapped the demon clan.
And then the issue, who freed the vampire clan and why? Perhaps fraudrin? If the seals are connected then perhaps it was a good chance to study the seal. And we can't rule out him using the vampires as a distraction to make his own move. Perhaps it is while the sins are away dealing with the vampires that he possessed dreyfus and starts his plot to get rid of the sins.
---------- Post added at 12:31 PM ---------- Previous post was at 12:27 PM ----------
Escanor, Merlin is always smiling...
Looks like he is still somewhat capable even in his Night form since he can dodge the attacks of the Vampire King, his Day form should obviously be stronger but he might be over 3000 at Night too, just without enough confidence.
I wonder what exactly is his curse.
Well, his changes seem to affect his personality too so it would be weird if he manages to keep his stats that high. He does not even have the strength to lift his weapon. And it'd be weird if in his kid form he has strong spirit... With that in mind I would think that at least his strength and spirit varies significantly through the day. His magic might remain the same though. Considering that it would be weird that his strength and spirit along with his possibly constant magic to go over 3000. We haven't seem him fight but it kinda seems like he will be the sort that fights mostly with his strength rather than magic. Basically a similar fighter to meliodas.