Access an account that was created as a “Network Account”

Not really - Lion is now able to kill processes that appear to be idle and unused. Fast user switching adds more load on memory and processor (due to the simple fact that more is now running), increasing the chance this will trigger.

Also, the changes to allow multiple simultaneous logins may have changed the API which will need the VPN client to request to stay connected or otherwise start itself outside the user space to survive a switch in Lion.

Well, I cracked it, it wasn't loginHooks I needed, but launchctl.

Basically, create two scripts (executable of course, you can make em root only too):

User1.sh

!/bin/bash ifconfig en0 up; ifconfig en1 down;

User2.sh

!/bin/bash ifconfig en1 up; ifconfig en0 down;

These of course make the assumption you're using an imac, en0 is the enthernet, en1 is the WiFi.

Then create a plist file (with root ownership, 644), mine was this:

User1's went in /Users/user1/Library/LaunchAgents/org.user1.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>Label</key>
    <string>org.user1.plist</string>
    <key>ProgramArguments</key>
    <array>
        <string>/Users/user1/bin/User1.sh</string>
    </array>
    <key>RunAtLoad</key>
    <true/>
</dict>
</plist>

User2's in /Users/user2/Library/LaunchAgents/org.user2.plist

 <?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
    <plist version="1.0">
    <dict>
        <key>Label</key>
        <string>org.user2.plist</string>
        <key>ProgramArguments</key>
        <array>
            <string>/Users/user2/bin/User2.sh</string>
        </array>
        <key>RunAtLoad</key>
        <true/>
    </dict>
    </plist>

After that run

sudo launchctl load -w /Users/user1/Library/LaunchAgents/org.user1.plist

and

sudo launchctl load -w /Users/user2/Library/LaunchAgents/org.user2.plist

as each user.

That's it, it's quick, simple, and if you set the owner of the scripts as someone else (and on another path), it's fairly secure (not bulletproof, but that's not in my scope at the moment).