This is perfectly acceptable. Also you could set resolution of such mask. For example in Quixel Mixer I am using 1k masks for 4k projects and it is enough (surprisingly you can texture terrain HM in Quixel mixer and it gives quite interesting results). Generally even upscaled mask is far more precise for tuning than vectors. Also if after singnificant upscale new details appear and you want to mask it, you have to do the same with vectors.