As much as I understand Vulkan API doesnt mean higher FPS in every situation and every rig.
If you have a low number objects scene ( high altitude , few aircafts and few ground units ) it should run almost other APIs.
If you have a high number objects scene ( low altitude, many aircrafts and ground units , high LOD situation ) Vulkan should give the best.
Can't really tell for the other sim but DCS has a more detailed terrain , high poly ground units and more chance to have a high traffic situation on air , land and sea.
Just imagine a Dynamic Campaign with populated airports , hundreds of aircrafts , SAMs , armors , civil traffic , ships , infantry units , etc.
Vulkan should give a huge boost of performance.