In reality what was the vision like for tanks to see behind them? I'm talking without someone up top of course.
I imagine they had rear vision slits/periscopes of some kind at least?
It would certainly make things interesting if they couldn't see behind them at all.
It really depended on the tank.
In Early Soviet tanks (super early model T-34 for example) had pretty limited vision. You had about 3 vision slits (if you were lucky) which faced out the front, one to the side and possibly a periscope that you could swivel to the rear (if you were a gymnast that is). There was a similar situation in tanks like the Panzer I and II and others. The Panzer III and Panzer IV on the other hand pretty much from their first iteration had a cupola (the big protrusion at the centre of the turret). It doesn't afford all round view, but it's great because it has little windows/ periscopes that allow you to look at areas all around and pretty much every turreted German tank had these cupolas. They must have been pretty indespensible because you see captured KV-1s which have had cupolas retrofitted to them.
It took a while for the allies to catch up with the cupola idea (though I'm sure they were around, I just can't think of any tanks which had them prior to the Germans).
Very late model T-34/76s had a cupola above the commander/gunner and all T-34-85s, ISs etc had them as standard. The Soviet cupola on the T-34 was actually pretty good because not only did it have the little periscopes dotted around the cupola circle, but it also had a standard all round periscope as well. Contrary to popular belief, both Soviet guns and Soviet optics were not as bad as is generally thought.
Early model Shermans likewise had a pretty much non-existant cupola, but with all later models (Easy 8s, possibly 76s, I'm not sure), they had the cupola as well.
In game terms this would translate as thus:
Tanks such as the T-34, and similar tanks without a cupola would have vision directly to the front and would have small arcs to the left and right, as it had a vision port there as well.
Tanks like the Panzer IV, which does a cupola, would have small vision ports all the way around, sort of like an 8 line cross, as well as vision to the front and to the sides from its vision ports.
Open topped tanks would have vision all the way around, but should be massively vulnerable to indirect fire (they aren't these days).
I'm not quite sure on the reasoning behind TDs and SPGs not having cupolas. Some did, (SU-100 is the prime example) but most (StuGs, ISUs etc) did not. Probably it's the fixed gun making it pretty much useless as also their more defensive role.
Likely in game terms these tanks would have vision straight out front, similar to the focus sight on SU-85 and JPIV