Here is something I observed while trying to book DL awards with VS miles. I found the flights I wanted on dl.com and then called VS. Tried to book DFW-LAX-NRT. The agent mentioned that the DFW-LAX segment was not available. I then tried just DFW-LAX on dl.com and it did not come out available at low level. So if you encounter such issues, it might be because of married segments.
I asked the VS agent to not do a segment by segment search and try searching DFW-NRT and that also did not come up available. I then went to af.com and that DL combination was not showing as available. So not only does DL have married segments, they allow them just with DL miles. I guess before calling VS, try checking af.com for the DL flights you want.