Currently not possible. Also in the same boat, purchased W on Delta = K on VS. K is not an upgradeable PE fare on VS but that doesn't seem to be the sticking point. I also purchased a round trip with VS S class (upgradeable PE) on the outbound and DL M on the return to burn a SWU. Even though the outbound is a VS flight and VS fare, because it was issued on 006 DL ticket stock VS cannot alter it in anyway.

The moral of the story is at present if you want to upgrade on VS you must purchase a ticket directly from VS in the appropriate fare class (see www.v-flyer.com for which fare classes are upgradeable).

The only glimmer of hope that was given after quite a bit of time with VS customer support was that there may be some wiggle room after you check-in, in which case VS can alter the ticket. I would expect you would probably need to buy up to S class in this case.

Hopefully the joint venture will become a little more cohesive with upgrades allowed from VS to DL and vis versa.