If there's a gold rush, sell shovels. I can see no other viable reason for Google to buy HTC. They surely don't want to manufacture phones. I can't think of any major patents HTC holds on the mobile arena, can you? There was a recent article about their phone division having financial trouble.
There's talk of spinning off the Vive portion of HTC.
Besides, the Vive is based on Valve's tech, HTC is mainly handling manufacturing only.