The Ballroom Thieves. Sure, they've got the classic upright bass and acoustic guitar, but they fall pretty far over on the alt end of the folk spectrum.
For one thing, they've got an enormous amount of energy that just shines both in their vocals and in their playing. In Vampires you can hear the guitar strings straining under the hammering they're getting but it's perfect for the song.
If you like Vampires, try Armada. Do let me know what you think, and big thanks to +Ferdinand Zebua for introducing me to them. I'm definitely going to be keeping an ear out for more.