How do I reformat paragraphs in vi?

[Steve Schossow]

I'm writing this with vi.  I know how to set my wrapmargin to get the
paragraphs to break properly when I'm entering text.  But what if I
re-arrange the text after entering it.  How do I get vi to reformat the
result?  Does someone have a macro that will do what I need?

Please no flames about why am I using vi, how I should be using emacs, 
etc..

E-mail is fine.  I can be reached as 'chip!ss at nosc.mil' I believe.