Sounds like a very interesting campaign, but I see a couple of problems.
As for there being a lot of Vampires around... I expect that they'd do a little population control of their own, taking care not to spawn more offspring than the local Human population can support.
There's no reason to think this unless there is some overlord controlling all the undead. If this non-daylight state is something recent, like in the last 30 years, the undead are going to go hog wild. None of them, especially not a bunch of solitary vampires are going to start analyzing food supply. As you alluded to, the wight, ghoul, and shadow population would be rising on endemic levels and every town would be like a scene from a post-apocalypse movie.
Perhaps the real problem for your party is that you've created an environment whereby you're able to justify something that isn't necessarily obvious to the other players. Now maybe they are expecting this to involve vamps and what not, but it's not immediately obvious that a cloud cover would protect a vampire. I don't recall vampires in movies coming out during the rainy season and hanging out at the local bar. Unless we're talking black as night cloud cover...like thick black volcanic-sulferdioxide cloud cover. And then I'd drop lots of hints that vampires and other undead were known to be protected by this during the day.
Another thing is that if someone has a vampire as a butler, with a +4 charisma modifier (which you bumped up), that's going to demand an explicit description to the party for the specific purpose of sending them a warning that something's up. Vampires are dominant personalities and I've never heard of them masquerading around as servants. A local nobleman..sure, a head servant? come on now. I don't even live in a world of vampires and if I saw a charismatic servant on the level you've described this vampire, I'd know something was up.
So the point I'm making is that your world is incongruous with the established paradigms and the players are paying the price. Personally, I think you're not playing fair, but that's for the party to decide and sometimes people don't really care.
The servant, seeing the Cleric staring at him, stared back and asked what the problem was. The Cleric, not being ready to face the guy down by himself at that moment, said he was just "lost in thought. Sorry if I started through you." The servant smiled, since their gazes had met, and I asked the Cleric for the Will save. His dice were cold and now he's the property of the Vampire.
Sorry, got to call "bogus" on this one if it went down as you described. How do you justify the vamp suspecting the Cleric? How does the vamp know the cleric is casting DE, and if he did, wouldn't he make sure to leave the area rather than be discovered? The idea that the vamp would immediately cast Dominate on the cleric without knowing a thing about the cleric and how powerful the cleric was or what his chance of resisting the Dominate is common problem of DM's who fail to set aside their metagame knowledge when playing creatures. If the vamp fails this Dominate...he's opened a can of worms.
I also have to call shenanigans on vamps dominating every person that suspects them, specifically in the situation you described. The main reason is that the more people you dominate, the more people you have to continue to dominate or kill.
Let me quote you the SRD on dominate:
Once you have given a dominated creature a command, it continues to attempt to carry out that command to the exclusion of all other activities except those necessary for day-to-day survival (such as sleeping, eating, and so forth). Because of this limited range of activity, a Sense Motive check against DC 15 (rather than DC 25) can determine that the subject’s behavior is being influenced by an enchantment effect (see the Sense Motive skill description).
This section is
extremely important for limiting the power of the Dominate spell and preventing vamps from taking over entire towns. This section can be interpreted as allowing lots of Sense Motive checks by various people to ferret out the problem, by anyone who comes in contact with the dominated person. The idea that a person could go around friends and family for days without someone suspecting something...in a universe were people know all kinds of stuff like this goes down...isn't plausible. You said the party members are 10th level? They would get a huge positive check modifier for sensing something is out of whack as they have been around the guy presumably since level 1.
The Dominate ability lets him spy on the party as he chooses, receiving sensory input from the Dominated PC (though not actually seeing through his eyes). That advance warning means that, if he has any brains at all he'll always be prepared for whatever they're bringing.
Such an interpretation would be an abuse of the rule. Dominate requires that the dominator is "concentrating fully" on the spell. So unless the vamp is in his room spending the entire day tracking every single person he's dominated (which he can't do) there's no way he's going to know what the party knows or discovers at the time they discover it, or, whether anyone else has figured out if one of his subjects is dominated. Nor will he be able to read the subject's mind to discover what they know. Your best bet is to command that person to ask the part questions while he concentrates. But that should trigger more Sense Motive checks by everyone in the conversation.
If the party came to me and laid out what went down as you've described, I'd call shenanigans. Now look, I realize that you're trying to make a fun campaign for players. I get that sometimes you have to fudge a thing here or there...but you're suggesting the players will end up losing and if so, it's because you've got to consider whether you've been playing this straight up.
What you've described, imi, isn't plausible. You also have consider whether the environment within which you've made this setting is consistent the reality of the situation. As an objective person, I'm not seeing it.
I'm not trying to call you out Greenie, but just saying you might want to rethink some of the things if you really think you've got the party over a barrel.