The only place where I could find the option was the game-select-menu when you launch raze. However that menu is only available if you have multible build-games in the same folder. If you just copy raze-files into your old dos-install-folders you will never find the vulkan option.
In gzdoom you can find it under "video-modes" or "display-options", depending on your menu-settings. I suggest you add the option for selecting render-apis there, in Raze too.
Congratulations on the 1.0 release!
