>assuming a file has been changed underneath vim, and I know about it, >how can I reopen that file replacing the current file's buffer contents?
In most cases when you toggle back to that window, it'll be smart enough to pop up a box and let you know that the file's been changed, and gives you 3 options, reload from disk, and 2 others (never used those, so didn't "stick" in my memory, as I only would reload), probably to ignore the changes and keep what you got, else cancel. Comes in handy when I'm vimming an error listing, delete lines as I fix the listed errors, then when I rescan/recompile/whatever, the error file gets rewritten out from under me. No prob, just bap the <cr> key when it pops up the box, and I'm back in bidniss to repeat for the next batch of fixes. Failing that, ':e' will attempt a reload, else ':e!' to force a reedit from disk.
