i don't think you will get a straight answer from vicroads.
you get an automatic exemption if you drive a probationary prohibited vehicle during work related activities, but you have to be able to prove its work related. depending on what the person is like that pulls you over, saying that you're doing work stuff could be enough, but more evidence may be needed. no evidence from vicroads is needed to say you can drive a probationary prohibited vehicle for work stuff.
this could sound funny, but you could get your boss (old man) to hand write a note saying that this is a company vehicle that you only have access to and use for work related stuff, and have him sign it too with a contact number.
if you're using it legitimately for work stuff, you wont get in trouble. i have been pulled over in my old mans V8 crewman a few times when im driving it for work, and nothing happened. just got asked if it was a work car and i said yeah its my bosses.