Open the output of a shell command in a split pane

With :!, the external command is executed in a shell, and the fleeting output isn't captured inside Vim; you just see what gets printed to the terminal, then control returns to Vim after the external command finishes.

To keep the output, you have to read it into a Vim buffer. For that, there's the :read! command. To open a new scratch buffer, combine this with :new:

:new | 0read ! <command>

If you want to pass the current buffer's filename (%) to the external command, you have to use :execute, so that it is already evaluated in the current buffer:

:execute 'new | 0read ! interpreter' expand('%')

Or, you use the fact that with a new buffer, the previous one becomes the alternate one, and use # instead of %:

:new | 0read ! interpreter #

Get rid of the scratch buffer with :bdelete!.

If you want to see the output asynchronously while it executes, you need an external multiplexer, or a plugin, as mentioned in @chaos answer.

As far as I know, you cannot do that in vim. But use a terminal multiplexer like screen for that. Before opening a vim start the multiplexer:

screen

Then press ctrl-a S for horitzontal and ctrl-a | for a vertical spilt screen.

Now open a file in vim in one of the split screens. And jump to the other screen using ctrl-a tab. Notice that, in the second screen you first have to open a new screen (ctrl-a c) before you can use it.

In the screen with vim you can edit the file and in the other screen you can execute it and see the messages and the code permanently.

Here a quick reference for screen.