It works out in practice that you have vi-like modes in emacs. When you hold control down you are in one mode and when you hold Meta down you are in another mode. The difference is state. In vi you have to remember which mode you are in. In emacs you know by which modifier you are holding down. The issue of compossabity is orthogonal.
For example in emacs you can move around, cut, paste, do minor manipulations like swapping words or case all while holding control down. When you want to enter text you let up.
For me, I was used to mainstream text editors that use modifier keys. I found vi difficult to get into, but I finally grokked it when I considered 'normal' mode to be affected by an imaginary key called 'ctrl-lock'. Like numlock and capslock, ctrl-lock keeps your modifier key 'held down'. It's not quite accurate, but it was the conceptual shift I needed to 'get it'. Also, you don't have to remember which mode you're in in modern vi clones, as they generally tell you in the status line. Not that you particularly need it, because when you're comfortable with any editor, you have a practiced set of moves that you don't have to think about.
Now I'm not a vim god, but I've used it enough that I have muscle memory that affects my use at the command line - I'll want to change something in a parenthesis and will type "ci(" (change inside parenthesis), only to find the literal characters appear... :)
For example in emacs you can move around, cut, paste, do minor manipulations like swapping words or case all while holding control down. When you want to enter text you let up.