How to execute file I'm editing in Vi(m)

There is the make command. It runs the command set in the makeprg option. Use % as a placeholder for the current file name. For example, if you were editing a python script:

:set makeprg=python\ %

Yes, you need to escape the space. After this you can simply run:

:make

If you wish, you can set the autowrite option and it will save automatically before running the makeprg:

:set autowrite

This solves the execute part. Don't know any way of getting that output into a split window that doesn't involve redirection to file.

To access the current buffer's filename, use %. To get it into a variable you can use the expand() function. To open a new window with a new buffer, use :new or :vnew. To pipe the output from a command into the current buffer, use :.! . Putting it all together:

:let f=expand("%")|vnew|execute '.!ruby "' . f . '"'

obviously replacing ruby with whatever command you want. I used execute so I could surround the filename with quotation marks, so it'll work if the filename has spaces in it.

Vim has ! ("bang") command which executes shell command directly from VIM window. Moreover it allows launching sequence of commands that are connected with pipe and read stdout.

For example:

! node %

is equivalent to opening command prompt window and launching commands:

cd my_current_directory 
node my_current_file

See "Vim tips: Working with external commands" for details.