1. Go to your Steam's Library and click Properties on Euro Truck Simulator 2
2. In 'general' tab uncheck "Use Desktop Game Theatre while SteamVR is active" and then click on "SET LAUNCH OPTIONS", type "-openvr" (without quotes) and click ok.
3. In 'betas' tab choose option "oculus - Experimental Oculus support (1.4.0)" and you should be good to go.
*note* Currently VR mode is a little bugged, which is not our fault, and when you launch ETS2 it might look like it's launched in Theatre Mode, but once you get inside the Truck everything should be in VR.
How corretly set-up properties look like: