There is a difference in bleeding and flushing. If you are simply flushing, the mityvac can get the job done, although AFIAC it is much more hassle than just finding a helper.

Bleeding is something else though. The MityVac often cannot provide enough volume to flow fast enough that the air bubbles do not rise faster than you can pull the fluid out.

On a lot of later cars with ABS, vacuum bleeding can cause lots of trouble. It's just best practice to use a pressure bleeder or the two man method.

If the mityvac works for you, then that's great, but beware that it may cause some trouble along the way.