Could you please share 2-3 screenshots of your Vray render that you think is normal and too dark/bright? So i can have a sense of your scene. Using only sun should be okay for this kind of interior case.
I think if you're going to include different seasons, it should be normal that renders will be darker (in winter) and brighter (in summer). For example, we often use artificial lights in winters in daytime. Thus, you should avoid tweaking your settings between scenes, in order to achieve more realistic result i believe.
I haven't used but this free plugin looks like measuring stuff you might need: http://sunhoursplugin.com/
(or this: viewtopic.php?f=323&t=53660
And here are some procrastination from me
I set this example on 23.June.2017 (UTC -4) for 9am-12pm-15pm .
Top gif is raw from Sketchup directly, below is rendered via Twilight.