The main plot is saving the citizens from the supernatural pests - not sure I would put this in the "romance" genre. How does saving the citizens jeopardise their relationship?
My biggest issue is you have supernatural pests, magic-wielding cowboys, vampires, and the Wild West. That's a lot of different components for an audience to get their head around in one movie and I wanna know why? Why does it have to be set in the Wild West? Why does the boyfriend have to be a vampire? Why does the cowboy have to wield magic? All of these elements have to be included for a good reason and be instrumental to the plot - they couldn't achieve their goal if they weren't a wizard and a vampire. The fact that it's in a tiny frontier town aids/hinders them how? This cowboy wields magic... how is some supernatural pests going to be a problem for him? Surely the vampire is only useful if the pests attack at night...? Do the townsfolk accept a wizard and a vampire living amongst them? So many questions!!!
Where's the arc for the cowboy? What does he learn? There is currently nothing that suggests he goes on a "journey". As Nicholas said, is there a way to make the threat more personal??
As a logline, I think it kinda hits the right notes, but as a story I worry that there's simply too much that you're trying to do.?Personally, I think it would be a better story if the cowboy and his boyfriend were just normal human beings. It's then an uphill struggle and a seemingly impossible goal that can unite the town. Maybe the result is that their gay relationship is accepted by the townsfolk. Most great films are comparatively simple - one man's quest to kill a shark, one man's quest to become a better father, one woman's quest to save her friendship, etc etc.