Originally Posted by xliioper

Only the marketing carrier matters, so it will earn based on distance flown for the return flight. You are booked on a DL fare outbound, and VS fare on return (due to VS marketing), but they coordinate fares and pricing as part of TATL JV. Inventory availability on individual flights is not coordinated, which is why you can sometimes save money by booking as codeshare marketing and partner fare.

Originally Posted by xliioper

If the VS operated flight is DL marketed (which I'm assuming is the case here), that's not going to apply to poster's situation as the above numbers will use fare paid in both directions of travel to calculate MQD and miles earned. Again, operating carrier is not relavant to earnings, only the marketing carrier code on flight is used to determine fare paid vs. distance traveled in MQD/mileage calculations. I suspect OP did not book through delta.com as it usually only shows DL codings and fares and does not give one the option to book as partner coding/fares.

Although it is sometimes possible to bring up partner marketed flights on delta.com if you use ITA Matrix Powertools browser extension (so perhaps OP used that). Base Z fares on flights below are $1820 each-way. JFK-LHR is roughly 3451 miles. So $1820 + (3451 x .3) = 2856 MQDs as shown below.