Purchased a flight to LHR on delta.com.
Flight is on VS metal, ticket is a 006, as expected.

Fare code appears as "O" on receipt.
O is the fare for Y award travel on DL, but also stands for discount-Y on VS (50% base miles).
As it turns out, I make more miles if the flight is credited using partner airline rules than the delta rules, so I'm cool with that.

Yet, I don't understand why delta would have used VS fare codes on a 006 ticket...

You don't mention the important element here: is the flight VS-coded, or DL-coded? A VS-coded flight is sold under VS fare rules, irrespective of ticketing carrier.

That is not clear to me and maybe I don't understand the issue.
If I call delta and purchase a ticket from them, isn't the flight always DL coded ?
I always thought that in line of principle I'm buying a ticket on Delta and the only difference would be an aircraft painted differently and FA's wearing unusual uniforms.

I mean, this time it turns out to my advantage, but what about other times when the operating carrier fare maybe doesn't give me any mileage ?
In addition, I can imagine potential problems with Fly America Act, if one buys a flight on delta.com and it turns out delta just seels him passage on a foreign carrier. Delta would be doing nothing differently from a travel agent selling that person a ticket on a foreing carrier, which is a violation.

No. DL makes this quite clear when flying on 006 tickets with AZ coded flights and lost baggage situation. DL can sell the tickets but the coding of the flights is what is important.

It is wrong to assert that codeshares violate the Fly America Act for US government funded travel. It has been the case for many years that if a code share flight is sold on the US carrier's ticket stock with the US carrier's flight number, it is considered to be a US flight as the code share process was deemed to be in the US commercial carriers' interest. Thus, it is irrelevant when flying Washington to Frankfurt whether the flight is on United or Lufthansa as as long as the Lufthansa flight is sold with a UA flight number on UA ticket stock.

No, on DL O means low business class award inventory, not coach.

Your understanding of partner mileage earning is incorrect. Since it is ticketed by DL, the flight will earn redeemable miles (and MQD's) based on fare cost, not miles flown for that flight. Only the partner MQM earning applies in this case (100% MQM's for VS O class). For redeemable miles and MQD's, the partner earnings apply only when the flight is partner-coded AND partner-ticketed. If the flight is either DL-coded OR DL-ticketed, DL earnings rules will apply and will be based on fare cost. Here are the relevant rules from the chart.

(1) Partner-marketed flights that are not ticketed by Delta will continue to earn miles based on a percentage of distance flown*
(3) Medallion Qualification Dollars (MQDs) apply to Medallion qualification. Flights marketed and ticketed by the partner will earn MQDs based on a percentage of distance flown*

Where are you seeing the fare class displayed?

If you're viewing your reservation on the VS website, the fare class you're seeing is the VS fare class, not the DL fare class (even if it's on 006 stock). The fare class that displays on the DL website should be different.

For example, a W (premium economy code share) fares on Delta could be W, S, H, or K tickets on Virgin. The DL and VS fare codes are not perfectly aligned.

And I did not.

On both Delta.com and email receipt.
The flight only has a VS number, as opposed to many others that have a double designator.

Yes, that could be it. delta doesn't care about fare code for RDM credit.
i was simply confused by the fact that for the first time I purchased a ticket from delta and it did not have a delta fare code. First time in 18 years of flying them.