This one takes some explanation, bear with us: A big part of what VSCO Film does is add contrast. Contrast by definition is making the dark areas darker and the bright areas brighter. If your photo is primarily dark, then VSCO Film's tone curves will probably make it seem like everything got darker. Similarly, if your photo is primarily bright, the reverse will be true.
Also, some films such as Portra 400 for Nikon need a significant Mid-range boost to counteract the strong mid-range contrast happening from the camera. In general, if your photo is properly exposed you shouldn't see a dramatic jump in brightness, but even "properly exposed" means different things to different people. To some it means not clipping the highlights. If this is you, then your photo is probably on the darker side and VSCO Film will seem to darken your photos for the reasons explained above.
To others, proper exposure means "Expose to the right" to get clean shadows, anticipating the extra dynamic range of raw highlights. If this is you, your photos are probably on the brighter side and VSCO Film may seem to brighten everything more. So, for us, not only is film a moving target but even people's starting point for exposure is a moving target! So, this problem is inevitable, but not unfixable.
If you are having problems with brightness shifts with VSCO Film, there are 2 courses of action you can take.
1. Change your shooting style based on what VSCO Film will do to your files
2. Keep it the same but know that you will have to adjust the exposure slider or highlights/shadows a lot.
If you're trying to match the look you've seen from a particular photographer, maybe even open up their photo next to yours and try to adjust the exposure to match.
Often we've seen people with brightness issues come to realize that either their screen was overly bright and throwing them off or that what they were aiming for in RAW processing was actually not bright or dark enough compared to the photos they were trying to match in their head.