session.custom

In the VPE, try to add "Logging" agent and add the session variable needed to log and apply to Access Profile.

If it's for troubleshooting you can try sessiondump from the cmd line, if the session exists while executing it.

It should absolutely show up, assuming it is being set with a value. You could also use a message box in your VPE flow to check for a specific session variable value. Example:

VPE:

start -> iRule agent -> message box -> allow

iRule:

when ACCESS_POLICY_AGENT_EVENT {
    ACCESS::session data set session.custom.foo "bar"
}

Message box:

%{session.custom.foo}