poterm just collects commands, executes them with the system() call, and displays the results in the window. As you saw from the messages, it is not a tty, so you cannot run stuff like vi that needs a real tty to operate properly.
I'm not sure what you mean by the command menu of poterm. Perhaps my old version doesn't have that. I pop up the keyboard by pressing the OK button (the round one inside the keypad). That is the standard way in most Inkview applications.
If you want something better and have WiFi available, you can use the utelnetd app for logging in remotely from your PC and getting a full terminal session that way. vi works nicely there.