I am running Fedora 8 and utilized the link below to get it to work. I know others used it with diff distros to get it working also.
Now if you have already done this, double check to see that you are using the right mic input (usually have 2 one on front header and another from output of card). Also check to see if Mic Boost is enabled and as you already mentioned capture enabled.
while running Vent, the Vent window must be active/foreground in order to toggle the mic. It will not pick up the mic PTT key if in background. (if someone found a fix for that, please let me know)