Well, to take the terms literally, vibrato is what you do with your fingers and tremolo what you do with the bar. So vibrato is a change in pitch and volume, and tremolo is a change in pitch. In guitar playing terms (i.e. not effects terms), vibrato is faster and much more subtle, whereas tremolo is slower and more obvious.

That being said, I have had limited experience with both these pedals. I can think of a couple of songs that use tremelo, and the above description seems fairly accurate. Since the two effects are similar, it would make sense that tweaking of settings would make them sound similar, just as overdrive and distortion can be made to sound similar with settings tweaking.

Alrighty, now that I've given you my opinion, I'm off to Google to find out the right answer!