Valve's Chet Faliszek has said pretty clearly in the past that Half Life 3 won't be a VR game, and I think that's pretty realistic. First person action based shooters don't translate well into VR. Any kind of locomotion that isn't walking around your room-scale space or teleporting just feels incredibly jarring, and it very quickly makes you feel motion sick.
So it'd be a pretty significant change to what we've come to expect from the Half Life games if they made it a VR title.
When you think about it, Half Life would actually be a prime setting for a VR game. The reason being that VR FPS work best when you can teleport rather than physically move (or move the camera through a continuous operation). Half Life always has had characters that can teleport, so it's not a big leap.
Additionally, HL3 will have to be big to wow fans, or it will be a disappointment. People have been waiting a long time, and riding the hype-train the whole way. The HL games were revolutionary, and, this would be another revolutionary step for them.
That seems like a bit of a stretch. While many of the Half-Life games have involved teleportation as a plot element, the vast majority of their gameplay has used more traditional methods of movement -- either walking or driving small vehicles.
I think that was a joke about the classic HL2 bug that lets you teleport NPCs from waypoint to waypoint during their scripted cutscenes to get through them faster.
So it'd be a pretty significant change to what we've come to expect from the Half Life games if they made it a VR title.