Getting Vim to recognize XML

Yep, :set syntax=xml should work. In vim 7.3, this sets :set indentexpr=XmlIndentGet(v:lnum,1).

If you've got a one-line file, you may need to :%s/\(<[^>]*>\)/\1\r/g, to insert newlines after every tag (or split it differently).

Then, gg=G should work.

Put

filetype plugin indent on

in your .vimrc to have Vim automatically identify .xml files as xml. You might need to put

set nocompatible

before that.

If the file extension is not .xml, you can make Vim threat it like xml by using

:set filetype=xml

After you do that, Vim's autoindention (and syntax highlighting, and omnicomplete (that in xml just closes tags, but that's still something)) will work properly for xml.