Reference check letters are merged differently than other mail merges, and must be done here. Trying to do this from any other tools will result in the volunteer getting the letter rather than the person providing the reference!
If you're creating a mail merge to be used for VSys Live online reference approval, don't actually send the letters now. But do use the Save settings link to save all of these settings - template, method, filters, etc. - for future use by a scheduled task.
You don't have to send reference check letters every time you enter a reference check request: you can enter many requests over a period of days if you want, then send the letters out in bulk here.