Return Line to Header or wing tank?

Assuming that the header tank has a vent back to the main tank, returning fuel to the header is just fine.
The header tank needs its own vent line or it will never be able to fill completely because the air will not have an escape route.
Return the fuel/vapor to the header tank to a point that is ABOVE the Fuel Withdrawal Port to avoid recycling the vapor bubbles back to the engine.

The 912iS return fuel line passes a lot of fuel and directing it to a header tank is more advisable than a wing tank as it may over flow due to all the fuel put back into it.