I should have been more clear, because yeah, it makes no sense to add both soda ash and MA at the same time!
I haven't added soda ash in a while (months), and was using it in conjunction with my old chlorine tablet usage days in an effort to keep my pH up.
I also wasn't very clear on my initial vacuum question. What's incorrect about vacuuming after adding MA to the pool, instead of brushing? I've got a 10' deep end and it's a lot easier to vacuum than brush (which I've never done, honestly).