The reference to the 11th hour means you should have applied long before the VR is set to expire, as in weeks ago, and not waited until the last possible moment on the last possible day.
Pay your fees online as "proof" that you intend to file a PR application once married and submit a copy of the receipt indicating you've paid along with the extension application. Also send along a wedding invitation which states the day, month, time, and location of the wedding, and indicate how many guests will be in attendance, the name and contact information of the minister who'll perform the marriage so they can confirm he's scheduled to marry you that day, the name of the photographer and his/her contact info for the same reason, things like that. Basically, you want to prove to them your intent to marry next month.