I have 2 Vizio's at home, they have no problem decoding the local Time-Warner clear QAM's, nor my 2 locally inserted QAM channels. Nor do the Vizio's I have here at work have any problems with TWC and the 8 locally inserted QAM's here.
However, The problem can easily be a combination of both. MPEG encoding and transports and receiver-decoder incompatibilities caused by software or local configuration issues can be finicky at times.
The views expressed here are my own and do not necessarily represent those of ABC, CBS, CW, FOX, MeTv, or AntennaTv; my employer; or its parent company.
Nor my wife for that matter!