Close.
You earn three points per dollar when buying airfare directly from an airline. No other type of travel purchase qualifies.
Membership Rewards points transfer 1000:1000 to Delta, but the transfer ratio to Virgin America is 200:100.
I see. I almost always purchase directly for my domestic travel. I've been hoarding AA points for international travel, no sense in burning miles on what's usually a cheap ticket. Therefore I'd most likely use the MR points on Delta or another big partner.