This is probably debatable, but I noticed that the AI will cease fire on a vehicle the second the last crew has exited the vehicle, whether or not they would've have knowledge of the vehicle's current crew count in reality.
It is a question of realism. The AI stops firing entirely if it has no other target in sight.
Prioriy should shift back to the vehicle if there's no infantry targets in sight.
http://www.youtube.com/watch?v=F0GcItentYY&feature=youtu.be
Here's a test I did of this. It's not terrible... they usually continue to shoot for a few seconds after you disappear but they don't make any effort shooting where they saw you last. I think in real life we would do that, keep them pinned down. At least smoke grenades work (see video).
Closing due to lack of activity in over five months. If this issue is still reproducable for you, please create a new ticket for it or ask for this one to be re-opened.