Not sure I fully understand your question.
"How can I see any PC's logging in via VPN?" - whether\not a system has connected via VPN would likely be a log of actions\events within that VPN solution
"Seeing" the systems can have multiple references. If the Altiris agent is able to communicate over the VPN session - then the agent's sync to Altiris server and the resulting session is what allows the system to be "seen". More on how the system is "seen" in this situation would be an Altiris specific question.
As to the vPro aspect of being seen - the focus is how to be "seen" outside of the corporate environment when inband mgmt\connectivity such as VPN is not available. In that case - the Fast-Call-for-Help would allow a system to be "seen" out-of-band when that system is outside the corporate environment. There are a number of posts on Fast-Call-for-Help on this site - one example is http://communities.intel.com/community/openportit/vproexpert/blog/2009/04/03/fast-call-for-help-video-demo--altiris-client-management-suite-v7-and-intel-vpro-technology