the diode job is because Vout connection raises and lowers voltages between 1.65v and 8v so the presence of vcc in that line will affect the behaviour of that circuit.
I don't follow.
So the voltage on the "output" can vary between 1v65 and 8v. I don't see what the diode does WRT that, all it does is raise the voltage the external device sees when the transistor is conducting, you will be getting about 1v5 on that with the diode and transistor voltage drop.
EDIT: Oh I think I get it, you are worried about VCC appearing on the output. Firstly it's through a resistor so probably not an issue depending on the relative impedances, but why is the resistor there in the first place.
I think we need more details of this external device.