On the mixer, using an ordinary Audio Track, set with "Input Monitor Mode" to 'On' or 'Auto', Armed for Record, or using an Input Bus channel, the signal ceases (is muted) whenever Vegas10 is not in Focus and the playback status is STOP; i.e. whenever another application becomes main focus.

This does not occur when VegasPro10 is in Play or Record mode.

The above did not occur in VegasPro9; I have checked.

Edit: found the reason:
Have to uncheck Preference "Close audio & MIDI ports when not the active application"

another casualty in transporting settings between major versions......