SteamVR Beta isn't necessary actually, Half Life 2 beta is.
I imagine it could be done on a devkit or Rift CV1 as well, it isn't reliant on anything Vive specific. From memory, this is how I did it:
In your Steam library right click on Half Life 2, go to properties, and switch to beta in the beta tab. Let it download if it needs to.
First, make sure you do not have SteamVR open, otherwise it will try to start it up in desktop cinema mode. What I did was start it up and then start SteamVR. However you can also just disable cinema mode in the properties for the game or the Steam settings, I just didn't know about it at the time.
Start up Half Life 2 beta, and in the options, enable virtual reality. If I recall correctly it will want you to restart the game, so restart it. In SteamVR make sure you untick direct mode (edit: in settings, devices dropdown), run in Extended instead.
Now what I did is started up Half Life 2 again, and then started SteamVR, and then clicked enable virtual reality in the main menu screen. You probably don't have to close SteamVR beforehand as long as you make sure it doesn't open in desktop theater mode.
That is how I got it to work. It was kind of finicky for me doing it the first time. Also make sure not to mess with the resolution options, but if you do and it stops working, delete the config.cfg file in /Half-Life 2/cfg/config.cfg and it should be fine again.
EDIT: If it isn't working, or you have a black screen when in-game try and delete the config.cfg file in /Half-Life 2/cfg/config.cfg