I think you might want to find a couple of careful beta readers to look your MS over before you start querying agents, because you may have a few story problems. You've got a whole lot of stuff going on here, and the query letter doesn't really create an impression that you've synthesized it into a cohesive narrative.
On a job to secure his freedom, a self-condemning bounty hunter tracks a member of his former gang to Gothic, a Rocky Mountain coal-mining town. The bounty hunter traded his death sentence for a job with the railroad company to bring in—dead or alive—the members of his former gang. He has two weeks left to find the last man or face execution instead of a pardon.
This setup doesn't make a lot of sense. They've let a convicted murderer go free to hunt down this gang. Even in fantasy, American courts don't turn murderers loose to capture other criminals. That's why we have cops and federal agents. If this story is set in a historical period when law in the West was less organized and the railroad company held a lot of sway, it still doesn't really make sense that anybody would want to turn this murderer loose.
And if this is a period story, why doesn't he just run for it? It's a lot easier for a fugitive to get away from the law in a 19th-century setting. He can just jump on a horse and ride for Mexico, which seems more rational than hanging around waiting to be executed, or dealing with a town full of vampire demons to catch this dude. If the setting is contemporary, it makes no sense that a railroad company could spring a convicted killer from death row.
What if, instead of dealing with the railroad company, he's dealing with the Devil? That at least has some sort of internal logic. If he doesn't collect these bounties for the devil, maybe he'll go to Hell in their place or something.
Also, do you actually know coal mining in Colorado? Have you done this research? If you have, you should find a way to concisely make it clear to the person reading your query that you know what you are talking about. If you do not have the background, a revision to the manuscript may be in order. I spent about two minutes looking up Colorado coal mining in Wikipedia and I learned that mines in the Rockies were especially dangerous, and that there were a lot of clashes between mine ownership and labor unions surrounding the conditions in the mines there in the early part of the 20th century. You'd have a much stronger hook if you built that history into your setup. Maybe your protagonist is an ex-con who is sent to this town as a strike breaker in the 1920's, and discovers something more sinister. Now you've got an original spin instead of what seems to be fairly generic angel/demon/vampires.
The bounty hunter’s chase is hindered when he realizes Gothic has succumbed to an assault by fallen angels, masquerading as vampires.
Vampires are hot. Angels are hot. Why not vampire angels? But I hear agents are getting flooded with queries in this genre. My understanding is that you need to bring an original spin and stellar execution to the table to break through. Also, the hot angels and vampires are sexy. Your story doesn't seem to have a romantic subplot. You should note that the sweet spot for vampires/angels/shapeshifters seems to be crossover YA stories where a young female protagonist becomes involved in a romance with the supernatural creature. Men are not buying books about fallen angels, and this book doesn't even seem to have any women in it.
The fallen angels, now demons, wreak destruction and fear while concealing their true identity. If they can cause doubt about their true existence as demons, then the world may doubt God exists, that Satan exists—leaving good and evil without merit.
Once you bring God into the mix, you may be turning your book into religious-themed fantasy, rather than general-interest fantasy. I honestly don't know if there is a niche in the religious market for demon hunters. If this is a key element you may need to limit your search to agents who specialize in the Christian market. Writing religious fiction is, once again, an area where you need to have specific knowledge. If you have a sophisticated background in theology you need to find a way to get that across in a query. If you do not, and you haven't thoroughly researched the subject matter before integrating it into your MS, you may want to consider an extensive revision. At the very least, you need a beta read from somebody familiar with the Christian market.
The bounty hunter, who is a former pastor and convicted killer, must face his past—the murder of his family, his rebuke of God, his crimes and his first meeting with the leading demon—as he tries to rediscover his self worth.
If your MC is a pastor, you really need to have done your homework. Also, a crisis of faith is essentially resolved once you confront the character with proof of God's existence. And if this guy murdered his own family, it's going to be hard to generate sympathy for him. It kind of sounds like you're trying to mix "The Shack" with "Buffy" and these are not necessarily flavors that are going to be good together.
In order to survive Gothic, the anti-hero must put his trust in a fallen angel, who shot him, and a college professor, who is a vampire folklore expert—while avoiding a vigilante mob of Hungarian miners bloodying the town as slayers.
Now here you've got this professor who is going to be a major character. First, it this character will dump off a lot of exposition, which is not a good thing, and, second, can you think and speak in a voice that's appropriate for this character? You're introducing another element that requires research.
There are pieces in place here for a good book, but based on this letter I'm not sure you've written one. Consider revising along these lines.
Even in good times, the remote coal-mining town of Gothic, Colorado is a kind of Hell on Earth, and these are not good times. It's 1923, and the miners here are fed up with working in dangerous conditions for slave wages. Union organizers have begun agitating among the rabble. The brass at the mining company is handling the situation the same way they always do; they've hired out-of-town muscle to break some heads.
None of them realize something even nastier is at work in Gothic; demons have secretly infiltrated both factions, and they're working to make sure the labor dispute turns into an all-out war. As the situation moves inexorably toward catastrophic violence, a mysterious bounty hunter arrives on a midnight stagecoach. He's an ex-preacher, wrongfully convicted of murdering his family. He saved himself from the noose by cutting a deal with the Devil, but working for Hell has turned out to be a fate worse than death. In Gothic, he'll have to make a choice: either he delivers the souls of the townspeople to the Devil, or he'll pay with his own.