Lync client 2010 and VPN firewall rule

Question

I am running Lync client 2010 (4.0.7577.4598) on Windows 7 x64. The server for this is a corporate one, and in parallel I use a Juniper VPN to access the corporate intranet and other resources. Everything works fine except that when the VPN is on and I use Lync to communicate with my coworkers with audio and video performance suffers a lot. I believe this may be due to the fact that Lync is routing the A/V over the VPN connection, which is not necessary since Lync works perfectly even without the VPN.

I have tried adding a firewall rule to prevent the Lync executable (communicator.exe) from opening ports through a VPN interface, but no luck, the rule has no effect.

Is there a way for me to force the Lync client to use the Ethernet or Wireless network adapter instead of the Juniper VPN virtual network adapter?

Thanks in advance!