I mean all those launch options like -dx9 -dx11 -vulkan -gl seemed the same, does the console provide any information about which graphics API the game is currently running?
You can run DOTA2 with the -vconsole option, then push tilde (`) to open the console. If you're using Vulkan it should say something like "Vulkan Command Buffer Pool Threshold" at the top of the console.
If the -vconsole flag doesn't work, you can manually launch the vconsole while DOTA2 is running at the following location:
steamapps\common\dota 2 beta\game\bin\win64\vconsole2.exe