Let pose the issue like this. You can do a VOR or NDB approach in a 777, i suppose and in a 737 with LNAV V/S or LNAV FPA or LNAV/VNAV, with the precondition of doing the altitude corrections if the temperature is 0 C or bellow. When you start to descend from the FAF the altitude you read at your altimeter will be the corrected. The next altitude , let say a fix or outer marker will be the corrected etc. until the corrected MDA or DA. You read on the altimeter the altitudes and verify if you are on profile or not . But you use LNAV/VNAV. You have pressed the LNAV button and the VNAV button haha. The same is if you do an RNAV GNSS approach. You use LNAV/VNAV. The RNP AR is an other story. I don't ask for RNP. At an RNP if you are not in the temperature or wind envelope you cannot do it and you do not do any corrections there. My question is why if you do the altitude corrections at the specific approach at Frankfurt and insert them in the FMS you can't use VNAV. You will correct the FAF REDGO,the 7.0 NM 2650', the 2NM 1060' . These are in the FMS. You will check if the 3 deg descend angle is there after REDGO in your FMS , you will validate the approach and you will descend to the LNAV corrected minima. With LNAV FPA you ' ll do the same thing. You will follow the 3 deg descend angle and you are legal to do it, but in LNAV/ VNAV you are doing the same thing but you are illegal .