Have you checked F availability? That might still be available even when Y is full (but at a high price).

Orbitz is showing, for Dec 25, DL having availability in coach, but for a high oneway fare:

$389 + $47 taxes & fees = $436 per person

Leave
Thu, Dec 25 Delta Air Lines 461
Depart: 10:55am
Los Angeles, CA (LAX)
Arrive: 4:01pm
Puerto Vallarta, Mexico (PVR)

You can book the DL flight right now, at http://www.orbitz.com or http://www.delta.com (it is positive space, all you need is a credit card, no need to standby at the airport).