If I understand the rules correctly, vehicular MGs can not be fired from any other hex than the first hex the vehicle fires from, even if the MA maintains ROF, and the only way they can be fired in the AFPh, is if the vehicle did not fire at all in the MPh.
But I guess a vehicle can fire it's MA several times before the MGs, as long as it maintains ROF and spends MP to delay in it's own hex.
That's all correct. The applicable rules are D3.3-.31 and D3.51 (last sentence before the EX). That last one is very easy to miss!