Thanks so much for your response, it's exactly what I needed. By changing the notification to use the sys_user table it allowed me to pull the first name by using ${first_name} meaning I can now use Parm2 to store the user's e-mail address instead of first name! 🙂 

In case anyone needs it in conjunction with the above post, here is my updated script for the VA topic that passes through the user to Parm2 (it is the addition of "vaInputs.user" in line 6 that is passed through to Parm2).

(function execute() {
var user = new GlideRecord('sys_user');
user.get(vaInputs.user);

if (user) {
    gs.eventQueue('sn_hr_va.va.email_transcript', user, vaSystem.getTranscript(),vaInputs.user);
}
})()