Hello,
It seems that your VPN don't authorize the mechanism you have implemented to deploy the agent.
Try to install the agent on on a VPN computer using this step:
1 - Log in using an account with administratives rights
2- Launch Internet Explorer
3 - Go to : HTTP://YourNSServer/Altiris/NS/Agent/AltirisAgentDownload.aspx (replace with your NS Server name)
4- Follow instructions to install agent using pull method.
Cordialy,
Stef.