Probably you have your Pixel Light Count set very low. I would recommend a setting of 3, which has still reasonable performance.
That would mean for every object VaM will take the 3 most important lights into account. Other less important lights would be calculated by vertex, which doesn't work well with hair as you noticed.
The setting @mspeck01 meant is here on each individual light source in the scene, for example an "InvisibleLight" atom. Just leave those on "Auto".
The setting does override the render mode for that particular light source, which is NOT a good idea. For some object in the scene another light source might be closer or brighter and therefore more important, so for that object you want that light as pixel light, but it can't if you forced the setting. If you share your scene with ForcePixel, you would override other peoples graphics settings, etc.