It's an interline partnership only. Much like how QF/UA or VA/AA, for example, allow interlines.Just discovered if you make a booking on Delta to an Australian destination, excluding Sydney they will sell a Virgin Australia ticket, with a VA flight number so you should get Virgin Australia status credits etc. Odd that they have have very few REX flights available considering they are actually partnered with REX.
Although no FF/credit is awarded on one leg unless a person is a FF of both programs (and subsequently has to retrospectively claim on both programs).
As for this DL/VA example, you can retrospectively claim VA credits/status for the VA legs only. Although the DL legs give out zero points/credits.