Thanks, that worked.
Here is what I found out about auto-focusing on an input field on pageload. The Javascript for the reply form is pretty easy to do:
window.onload=function() { document.forms['vbform'].elements['title'].focus(); }
Unfortunately, it breaks the Backspace key to move back in your browsing history. By focusing on a form field you're essentially disabling some fundamental browser functionality.
Kovid, when you are in Reply using vimperator, instead of using an extra 3 keystrokes to focus, you could use the tab key instead. That would reduce it to 1 extra keystroke.