I just dropped in one of the w(q)vga ones from here and it worked fine. I think those are 800x480 which is smaller than the vox's screen.
http://forum.xda-developers.com/showthread.php?t=696193
I was getting a little tired of the kobo animation. (not nearly as tired as I was of the startup sound when I blew it away a month ago...now I can't even remember what it sounds like!)
[edit]: Ended up going with AndroidParticles2 from here which is pretty sweet:
http://forum.xda-developers.com/showthread.php?t=905538