Yes, VA domestic should offer DL DM (and only DM) free EconomyX. But this is only for domestic flights. Since this thread is about international flights, it's just important to note that.
You can't even purchase EconomyX on the VA operated DL ticketed international codeshares. The problem is that since it is a DL ticket, DL would have to take the revenue from EconomyX sales, and they don't have the IT in place to do that. So it's impossible (except possibly on day of travel). Since there is no charge on domestic routes for DM, it is possible, as the issue of how to charge goes away.