Unless HTC are on the verge of collapse it wouldn't make sense for HTC.
Remember HTC make their money from hardware, Google from software and ad revenue...
Purchasing a hardware company and putting out only 1/2 handsets a year as Google seem akin to do (and even then almost reluctantly) just doesn't make any commercial sense for HTC to be bought out.
Google may be happy to partner with a company and let them build their device, but that's very far away from wanting to step into the full hardware game itself. It's already demonstrated that it doesn't want to do that, and unless HTC were financially on the ropes with the threat of going under imminently then it would not make sense for them either.
HTC had great success with Vibe, it may be simply a case that HTC's future will continue to be branched off as they develop new hardware and eventually smartphones may play a very fleeting part of its overall portfolio.