Chilump:
Your vizio TV tuner can tune QAM channels (unencrypted ones). It doesn't need any special drivers, it has everything it needs inside the TV.
Windows VISTA (and MCE 2005) cannot tune QAM channels with its media center BDA driver regardless of what tuner you use. Your tuner card you bought says QAM on it because the tuner company wrote special software that allows it to tune QAM channels in windows, but the catch is you have to use their software, not windows media center.
Microsoft has not given out a 'standard' approach on how to write QAM BDA drivers, so basically no tuners on the market can tune QAM channels through windows media center. HDHomeRun tricks windows media center into thinking that it is different type of tuner altogether (ATSC antenna), so it doesn't need a QAM capable BDA driver.
Someone correct me if I'm wrong, but I am under the impression microsoft has not given much effort in getting QAM working because of the pending cablecard/OCUR boxes soon to go out.