VPN access through proxy

[macVS]

hello everyone,
I have a problem while working on a customer with my macbook pro.
The customer adopts a proxy and I have to connect to my company vpn. Everything is ok with a Windows pc...but with my new Mac I'm not able to connect to VPN.
Let me explain:
I connect to the intranet lan and set the proxy. Everything it's ok and I'm able to connect to customer intranet and Internet.
Now, to connect to my VPN, I use IPSec, I have configured the VPN with IPSec with my company VPN address and credential, and set the proxy in the tab of the advanced options. But when I try to connect to VPN, no connection happens! If I try to ping the VPN ip address...it fails.
I know that all people working here with a Mac have a similar problem!
I've tried to use ntlmaps, authoxy...but with no success!
Any ideas?
Thanx in advance!
Valerio