They have been around for awhile on DL marketed, VS operated flights. I don't believe they are available on VS marketed flights. They will rebook into VS G class which is an award booking class and you will have to request retroactive mileage credit as they won't credit automatically due to the booking class. You should be able to check for availability on flights by checking for VS awards on VS website (if there is VS award availability in UC, it means that G bucket is open on flight and you should be able to apply MUA as long as you have booked into eligible DL fare class on it).

Note that Terms and Conditions are found on Delta website. You need to expand the T&C section under the "Upgrade with Miles after Booking" section.

Definitive Thread: Use Sky Miles to upgrade on VS
https://www.delta.com/us/en/skymiles...rades-and-more