So these resources are what the OP was about. What resources should there be for players to use in play?
Like, what would make the players suspicious? Is it GM cues in playing the patron NPC and other NPCs? Is it the ability to leverage contacts and allies in the game world? Class abilities and the like?
Could be any or all of the above, depending on the situation. The patron might let something slip or do something to arouse suspicion (obvious example: a vampire pulling back from walking past a mirror on the wall), but more useful would be the leveraging of (and, in the case of a new campaign, the making of!) contacts and allies. Class abilities could also come into play, again situationally dependent.
But they key thing is: the players/PCs have to a) be or become suspicious and b) act on it! And for a), they sometimes need to be or become suspicious even if there's no obvious reason to do so, as in "this is too good to be true".
But it's not just like real life. It's a game, and so it has to be considered as a game.
It's a game in which I prefer to try and simulate real-life interactions where possible.
No matter what, there is a relationship going on between the participants, and how the game goes will impact that relationship.
So, for instance, the betraying patron trope may wind up causing me to mistrust any or all NPCs that the GM puts in front of me. That's going to impact the game.
Now, it may not cause me to do that. But my point is that the risk is there, and the benefit I seem to get by having them betrayed in such a way... which, I should add, in your original example went even further by casting ALL the accomplishments of the PCs in a new light, that of having helped the vampire achieve his goals... is that they know there are NPCs they can't trust, I don't see that risk being worth this reward.
"Hey, everything you guys have done? It's actually been in service to this vampire because that's what I designed!" is not, in my opinion, a way to build trust between players and GM.
There's two very different types of trust at work here, one of which I care about building and the other of which I'm quite happy to destroy.
1. Out of character, do the players inherently trust me-as-DM to run a good, fair, and fun game? Ideally, yes.
2. In character, do the players' PCs inherently trust anything I-as-GM put in front of them in the setting or fiction? I don't care.
These two things should not impact one another.
So what were the clues that they missed in the second example? What kinds of resources were available for them to potentially learn of this betrayal beforehand?
Beforehand, none, as this was the very start of the campaign. And I'll pull in
@Campbell's post here, as it'll save typing the rest of this twice.
Campbell said:
When it comes to betrayal I think it's really useful to ask why an NPC would betray the PCs. What do they gain from it? Why were they invested in the characters in the first place? What precipitated the face-heel turn? Betrayal tropes in fiction often do not make a whole lot of sense. Beyond that what does betrayal add that asking the player characters to do increasingly questionable stuff does not? That's often more sensible fiction, better storytelling and better gaming.
It would take ages to present the whole story, but the basic scenario was that a long-lost but famous adventuring Company was in town and holding what amounted to a job fair for neophyte adventurers. Each of the players had to, during roll-up night, come up with why their PC(s) would be at this job fair, and the campaign began at the start of a big meeting of all the "applicants".
The applicants were all signed in, then divided into parties (with of course all the PCs being assigned to the same party). Each proto-party was then given what seemed like a simple field-trial mission, and sent into the field; the PCs' party's mission was just to explore and map a supposedly-empty castle/dungeon complex not far from town.
The betrayal: the "Company" was all fake (the real Company disbanded years ago, a long way from here), and the "missions" had been set up as thinly-disguised suicide runs. The complex the PCs had been sent to map wasn't empty at all (a major hint that all wasn't as it seemed!), but they persevered and managed to (mostly) clear it out.
On getting back to town it soon became clear things were a bit off. Of the seven parties sent out, only theirs and parts of one other had returned, and their contact was (blatantly!) surprised to see the PCs when they arrived. On this they started asking questions etc. around town and fairly quickly realized they'd been had. A very long story then followed........
Reason for the betrayal: the "Company" had grand plans for the near future (including overthrow of the throne) and as part of their long-range preparation wanted to strip the city and region of all its neophyte adventurers before they could grow up to become significant threats.
I'm really only concerned about the setting in how it relates to my characters. I don't care about the pages and pages of notes the GM has written until they matter to play. And I say this as a GM who used to spend an inordinate amount of time ahead of play on worldbuilding. More often than not, the vast majority of that effort is wasted. Sure, now and again, a GM will create a cool bit of setting detail that will make me go "ah, that's clever" or similar. But unless it leads to something that is playable, it doesn't really matter.
Thing is, even if those pages and pages of notes never come into play, their existence provides a sound framework for those parts which do. Further, what isn't used now can always be saved for later, particularly in a long-running and-or sprawling campaign.