I think it would be cool to have a vi/vim engine that can be added onto editors. The editor would only have to overload simple functions like keyPressed(key), getChar(pos), setChar(pos, c), etc, and the engine would take care of the rest of the vi/vim emulation.

This approach would be better because there will be a single effort rather than each editor writing its own vi/vim mode. Also when a new feature is added to the engine it would be available on all editors.