The first time you launch the game, you'll be given the option to play in Steam VR mode.
If you mis-click or add VR at a later date, go to Library > Le Mans Ultimate > Properties > General and then change the Launch Option from "Play Le Mans Ultimate" to "Launch Le Mans Ultimate in Steam VR Mode".
In game - you should map a button to "VR Centre head position". Additionally, you will find the ability to change scaling for VR in the "Graphics" menu - HUD, Menu and World scale are all adjustable.
Discovering the best settings for visibility vs performance on your PC and resolution may take some time and experimentation.
Some users prefer to play with runtimes other than SteamVR and if you search the internet and YouTube you will find instructions on how to use these. In particular there is a passionate community found on our Discord in the #lmu-vr channel.