Board Thread:Lore Discussion/@comment-8931766-20130515212016/@comment-1336446-20140417030301

Twighlight jokes aside, my interpretation is that they are "hunting trophies" if you will, real serial killers sometimes do the same thing.

Or... vampires are traditionally obsessive compulive. In fact one of the defenses in legend is (believe it or not) to throw a handful of seeds at them as the vampire will be compelled to stop and count them, giving you time to get away. Maybe collecting boots is an expression of that among Skyrim's vamps.