This isn't a file per se, but it is some secret vim-fu that lets you insert unicode and show it, and null characters:
First off in Vim if you see ^@ it is a NULL character. To type it, get into INSERT MODE and then press Ctrl-v, Ctrl-2.
Tada!
For entering in an html escaped unicode character, like the ones listed here:
http://www.amp-what.com/unicode/search/checkbox
Click the U in a circle on the right, and it shows for a checkbox: U+2610
So to type this in vim: go into INSERT MODE, then type Ctrl-v,u2610
Tada! Vim sometimes gets confused by Utf-8 at least when I was testing it. Sometimes it would chop the rest of the line visually after inserting the unicode character (the next time you load the file).
To switch the encoding on the file, there is this:
http://stackoverflow.com/questions/778069/how-can-i-change-a-files-encoding-with-vim
:write ++enc=utf-8 russian.txt