By default, VIP makes the standard VI keys available in Insert mode. But this also means that Emacs major modes cannot be used in Insert mode. It is strongly recommended that as soon as you are comfortable, make the Emacs mode bindings visible. See section Customization, to see how to do this.
Once this is done, it is possible to do quite a bit of editing in the Insert mode, and also use Shell, Dired and other modes of Emacs.
You can call Meta functions from Insert mode. The Vi to Emacs toggle key, normally C-z, can now be used as Meta. Thus M-x is typed as C-zx. _ will now insert itself.