Why don't I see more progress when sanding my stair treads?
A vibrating or oscillating sander isn't going to be aggressive enough for that job. It's really only suitable for light finish sanding. You need something that spins, or at least something with a random orbit (more movement).
80 grit is probably a good choice for working through the varnish on your steps but you need the moves only a different type of sander provides or you'll grow old waiting. Part of the problem is that varnish clogs paper grit quickly--it doesn't let go like clean sawdust does.
You might also consider chemically stripping the varnish to hurry the restoration process. Sanding through it can be a real chore.
All that said, if you have the option of temporarily removing your treads, that's what I'd be doing. You'll have a difficult time getting satisfactory results around the edges with them installed. You'll have visible lines from the existing varnish and stain, and you may have visible surface level variation as well. If you can gently pop them out, pull the nails out the back side, and sand or strip fully in a workshop area, that's the ticket.