[Imp]

Whats bigger infantry or the vehicle?
The vehicle therefore more chance for stray shots to hit it & you hear them.
Sharing a hex with any unit but esp a vehicle means the units are more likely to be suppressed due to other units in the hex attracting fire.
Given the choice if all other things are equal I will always fire at the hex containing both infantry & a vehicle rather than a single unit. I get extra suppresion possibly damage for free meaning the second unit in the hex often stops firing early due to suppression which makes things a whole lot easier.