Optical vignetting is the darkening of the corners of an image compared to the centre.
A DX lens projects a smaller image circle than an FX one as it only needs to cover a smaller sensor. This makes the lenses smaller and less complex to manufacture, This smaller image circle means that the corners on FX are black or nearly so.
To eliminate the vignetting and sharpness falloff in the corners, the D750 has a DX mode. This will crop the image to a DX sized part of the sensor. This leaves only 10.7 megapixels to work with.
For cheer competitions, wouldn't a D500 be a better camera than the D750? It is, after all, made for shooting fast-paced sports. Can you get close enough that the 70-200 has enough reach? 200 mm on DX gives the same angle of view as 300 mm on FX or, the other way around, 200 mm on FX is equal to ~135 mm on DX.