0 A.D. is an in-development free and open source RTS, one that's very promising and looks pretty good and the next Alpha version will also feature experimental Vulkan support.
Performance has been a problem for 0 A.D. across many versions, so I'm quite hopeful that this will help towards that. They put up a very brief announcement, which links to a forum post going into more details. It's currently going through testing, with the developer asking people to give it a run (although you would need to compile it from source) so that they can get Vulkan into a stable state ready for release. However, a Release Candidate should be available sometime soon to make that easier.
Don't go expecting better performance yet though, as they need to work on the multi-threading. Right now they're focused on ensuring it works and doesn't run into driver problems.
See their previous release trailer below: