One reason your getting different answers is the round up rule for 800 and less on feeders.
I'll try to explain a little. Since the conductor size is not listed. You use what you know or ask the designer.
You know the OCPD is 800, you also know the load and this is a feeder.
So in reality you have to do two things. The conductor(s) has to carry the load and be sized to the OCPD according to the NEC.
Two rules apply 240.4 and 240.6 in this case.
Other do as well however these two help in this case.
Since the load is under 700 and the next standard size for an OCPD is 800. Your conductor needs to fall with in this range unless your 800 amp is an adjustable circuit breaker and your on the 2023 NEC. Than a further reduction in conductor size can be used.
We're over 100 amps so 75 c is you choice in the conductor column for the location the conductor is installed in.
Start there and size. This is one reason a parelle set of 500 works in your case.
380 x 2 = 760. In the field we call it an over under. Over the load (760) and under the OCPD (800).
Now if you want you can do the VD if you want not mandated by code in this case only recommend.
Use your parelle 500s and the calculated load to see if these work for your VD.
A sinero:
What one can do based on 2 - 2" emt raceways and and adjustable 800 amp breaker is change the feeder to a 400 amp or 2 sets of 3/0 and dial down the breaker. Now you meet 240.4 and 240.6. Then run you VD on that if you want.
Not saying this is what I would do in your case just an example of how the rules work.
400 amp conductors on a 400 amp OCD. It carries the load.
One can submit that as an option based on what's provided. Or get a change order to size raceway and conductors to the designers choice.