This problem has subsequently been solved by locking the volume control to TV mode. "Interesting" that neither Comcast nor Vizio people suggested this. My nephew fiddled with it and figured it out.
Have coded-in my new Vizio E321VL LCD TV to a Comcast 3-in-1 (Platinum with red OK button) remote control via Comcast's Motorola RNG150N cable box. All functions of the remote work perfectly, with one exception: the volume button. When in the default Cable mode and attempting to raise or lower the volume, the cable volume bar comes on-screen and will move up or down accordingly, but the volume itself doesn't change. The volume button only works if I press the TV button and get into TV mode, in which case I'm then using the TV's volume control. Have gone through two different cable boxes and remotes, with the same result. Have tried all of Comcast's Vizio codes. Vizio's customer service suggested I also try Goldstar and LG setup codes, which I did, to no avail. Have had Comcast tech here; he couldn't figure it out. There's no other equipment hooked-up; just the TV to cable box via HDMI. I would like the volume button on the remote to work in the default cable mode. Any possible remedies or suggestions will be appreciated. Thanks.