Have you applied the startup modifiers (there's a box you need to fill in in properties for the game, you add something like -vr to it, this may be the wrong phrase) and downloaded the beta updates? You need to have installed the Steam VR beta drivers AND the beta version of HL2 (opt in via the properties menu for the game).
Once all thats in, you can put steam into VR mode and start HL2. If HL2 doesn;t start in VR, you may need to enable it via the in-game options. I forget which menu its hidden under!
Apologies if you already tried all that, if it doesn't help then I am a bit stumped 😞
I got my DK2 working on my notebook with switchable graphics (6770M 2GB) and with the help of a step-by-step I got Steam VR and Half-Life 2 working perfectly.