I'm Delta Plat and have a Plat AMEX (if that matters these days). In Oct, I'm taking the Mrs. (no status) to Maine for a long fall weekend getaway to celebrate her 50th. I want to show her that all my points & status strategery matters with first class seats for her and I. Yet, I'm admittedly a cheap b.

I plan on using a travel credit voucher and miles, plus, maybe some cash to buy the RDU-BGR/PWM standard economy tickets, which are $700 each (ouch! again, I'm cheap).

Question: what combo of purchase methods (voucher, miles, money) will have the highest probability of first class upgrade for at least one ticket, and preferably both?

BTW: I usually fly RDU-DCA and get 1st class upgrades off standard economy for 90% of my flights. Granted, I'm out early and back late, so maybe demand/competition is minimal then...