ok, so i think i just figured out a work around, but i don't like it very much.
if i set the alarm to go off with a sound, then toggle the ringer volume switch on the side of the phone all the way down, then the alarm will go off with just vibrate. but then i have to remember to toggle the switch back up. not cool.
so i think this may qualify as a bug. don't you?
how do i ask apple to fix this in the next update?