Have you tried using the logging module? Might be overkill for really small scripts as it requires a few lines to set up, but even the tiniest tool ends up requiring print-debugging at some point. With logging it works the same in 2/3, you can leave all the debugging statements in, and if a library also uses it then you get additional debug output from that too. Add a "debug" command line flag to pass when something goes wrong down the road (to change the default log level), and you don't have to add/remove statements for debugging.

For a vim extension - I use snipMate http://www.vim.org/scripts/script.php?script_id=2540 It's a bit on the dead side but works great as-is. ifmain followed by the expander key (shift-tab in my case) expands to the if __name__ ... idiom. You could add your own "p" snippet that expanded to print() in python code.