DM/PMs can now use Certificates to upgrade on a Delta Vacation flight (that before was quite rare to get it booked in a fare class that allowed it.)
The news is quite recent and it took me a few tries to the Medallion line to get an agent that didn't question the policy change (one had me sent the Delta URL where I was seeing that, as "I've been here a long time and we never allowed that").
Once reaching one with knowledge of the change, the process was the same as applying it to any other flight.
delta.com/us/en/skymiles/program-resources/news-and-updates (scroll to Recent Medallion Upgrades).
A New Way for Diamond and Platinum Medallion Members to use Upgrade Certificates
Upgrade Certificates, which are available exclusively for Diamond and Platinum Medallion Members as a Choice Benefit, may now be used to upgrade on flights that are booked as a part of a Delta Vacations® package.
After a quick search, I didn't find it posted around here yet - so e
njoy!