WLM works only with my VPN connection

Hi,
I have a problem using my webcam with WLM 2009 (on Vista 32bits). My webcam
always worked fine with WLM but since I have install a VNP connection,
something strange appends:

It works fine when i use the VPN connection. But, when i use my home
connection, my webcam doesn't work. In fact, it works only when my contact
uses WLM 2008 (or an earlier version) but not with the 2009 version. I can't
fix it. I have try most of the solutions I could find on internet, without
success.

Can anybody help me?
Thanks
Marie