Hello Walkuere. I have the same headset and it is unfortunate that you are experiencing problems with what I consider a great headset. I am a loyal customer to logitech when it comes down to PC or PC gaming accessories.
I have the same issue you get from time to time. Nothing with your heatset but I think its the software it uses. The solution to your problem is that you have to switch it from surround sound to normal stereo via the switch on the left cup of the earphones. After that you should be able to hear everyone in vent. Now this isn't a permanent fix because its a bug from the software. I only do this to tell the people in the channel I am in I will be right back. Also this only happens with Call of Duty: Modern Warfare 2 from what I have noticed. So all I do to fix it is exit the game. Close Ventrillo. Start Ventrillo back up. Start game back up and verify I can hear vent and they can hear me. Works every time. I have also sent an email to Logitech about the issue and hope they figure it out and have it patched up soon.
If this doesn't help then I am afraid you have a different issue but feel free to PM me since we both have the same headset.
Also in regards to your statement of (the game we aren't supposed to discuss) if you are referring to Modern Warfare 2 here is an update you might have not seen. Here is the link
http://edgegamers.org/forums/showthread.php?t=109060 and the main message below this text.
"In regards to the boycott of MW2 and our eGO members:
Basically, a few people ruined it for the rest of us. Immature posts, flaming, people ignoring the chain of command...
Seriously... how much sense does it make to use up leadership time and eGO resources to settle or even have arguments about this game when there are no dedicated servers. (eGO bread and butter)
It was disappointing to have to go this route but even more so that it was necessary.
Edgegamers wants nothing to do with people in the community getting emo about a game that isnt eGO friendly.
That being said, if you have any questions about MW2, just like with any other non divisional game; feel free to use misc. game discussion or the tech help threads in the general forums. Should any type of flaming, arguments, immaturity or disrespect of any kind rear its ugly head; that's probably the last you'll hear from the guilty party.
You may wear the tag if you choose. Keep in mind whether you do or don't you are expected to act accordingly.
You may not post in any way, shape or form: MW2 in your vent comments. Not even a hint.
If you have any questions please direct them to the CoD Division leaders and advisors in forum private messages.
Hope that clears up some of the confusion and any feelings of being alienated."
Good Luck I hope this helps you out.