I think it's the opposite. You can make the MG fire as long as you have hold fire activated when upgraded. If you toggle it, prioritize vehicle will affect the MG as well.
No, there is a way that MG will still fire even when vehicle priority is ON.
I will test it tonight.
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Ok, it;s totaly fucked up.
If you turn priority ON and then upgrade MG, the MG will shoot at infanntry.
BUT
If you turn priority OFF and ON again, the MG won't shoot at infantry anymore, so turn ON, upgrade and keep it - gg lelic.
So like you said, but one more thing:
If you have priority ON and manually order to attack infantry, main gun will shoot but the MG will not - even better gg lelic.