Different Frequent Flyer Program for Certain Legs on Same Itinerary

[Copernicus]

I couldn't find a thread for this.

I have Platinum status with FlyingBlue and regularly come into situations where some legs on my itineraries with partner airlines are in a fare class that rewards zero miles. 

Today I'm flying Vietnam Airlines and my outward flight rewards miles but my return is booked in discount economy and rewards no miles with FlyingBlue.  So I thought I should be signing up with these airlines that I fly discount economy with, but I still want to use the lounges as well.  As far as I know, once my frequent flyer number is entered into the itinerary, it remains there for the entire itinerary and trying to enter separate frequent flyer numbers for each leg wouldn't work or would just confuse their systems.

Any experience or advice for these situations?

[mileagejunkie]

I couldn't find a thread for this.

I have Platinum status with FlyingBlue and regularly come into situations where some legs on my itineraries with partner airlines are in a fare class that rewards zero miles. 

Today I'm flying Vietnam Airlines and my outward flight rewards miles but my return is booked in discount economy and rewards no miles with FlyingBlue.  So I thought I should be signing up with these airlines that I fly discount economy with, but I still want to use the lounges as well.  As far as I know, once my frequent flyer number is entered into the itinerary, it remains there for the entire itinerary and trying to enter separate frequent flyer numbers for each leg wouldn't work or would just confuse their systems.

Any experience or advice for these situations?

In that case you'll have to go to a desk at every step of the way you want to change it. As for the lounge, ask them to change it to the new ff# as you leave the lounge.
Also make sure to have them print a ticket or itinerary with the new number showing as it doesn't always go through and they have to try again.