Just get the VN PNR from Delta (every agent can see it, if you get a incompetent or just plain lazy agent who refuses to give it you, HUACA until you get someone who will), then go to the VN site and select the seats. Same is true for all SkyTeam airline bookings (though for some like KE, KL, AF, you can select seats on delta.com and those will actually stick - usually).