If you turn on the edit cage for the mirrored side, then if you try to move the mirrored side vertices, they actually behave as if you were trying to move the non-mirrored side, i.e. (assuming you're mirroring the x-axis) you have to move the mouse left if you want to move the vertex right and right if you want to move it left.
VERY confusing and super annoying. Is there an addon or something that changes this behaviour so that you can move the mirrored vertices as if they were actual vertices?