If you read the support page on the website, you'll see that it's a FS limitation probleme but if you want a darker cockpit, you can edit the aircraft.cfg and remove the "dome" light effect.

and the aircraft will be more dark ! :) Captain Sim work on a new system for the night.

Please read some time the support part on the registered part of the site ! Many interesting informations :

Example :

Q: I can't adjust the the panel lights, they are either on or off or full bright.

A: Three VC lightning options available:
- Lightmaps Off
- Lightmaps On
- Instruments Backlight On with Lightmaps On

As for the lightmaps, some people like dark cockpit, other like bright one. But MSFS works with just one set of lightmaps.

If you want your cockpit to be darker, you can turn off (comment) the VC light effect (#8)

Aircraft.cfg

[LIGHTS]
...
//light.8 =4, 68.48, 0.00, 5.19, fx_vclightwhi
...

We'll consider the possibility of another 'darker cockpit' set of lightmaps.

From: 757 Captain (FS9) / Virtual Cockpit (graphics) (QID 487)

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ps : On FSX, the lines are not the 8 but the two lines :

light.2 = 4,  67.00,   2.00,  5.19, fx_vclightwhi
light.3 = 4,  67.00,   -2.00,  5.19, fx_vclightwhi