tremolo is volume modulation and vibe pedals offer a bit of pitch modulation and chorus. i THINK sometimes 4-stage phasers can achieve vibe sounds.
i know you're asking about vibe pedals, but sometimes trem and vibrato get mixed up too. old fender amps labeled with a "vibrato" channel actually were tremolo channels.... i'd imagine that's where some confusion started... vibrato is definitely pitch modulation.
Indeed, Leo Fender did mix the terms up..the Strats trem is actually a vibrato unit and almost all Fender amps have tremolo, not vibrato.
The Univibe was developed as a rotating speaker simulator for organs. On organs, it's fairly convincing, on guitar, it just doesn't have the same complexity as a real rotating speaker, but it has its own unique sound that has become legendary things to Jimi and Robin Trower.
Univibes have two sides...Chorus and Vibrato. The vibrato is really pitch shifting vibrato, but it's not the side everyone uses, that's the "Chorus" side, but it should be noted, that the term "chorus" is sort of an earliest use of the word, it is not the same flanger/delay-based effect that was introduce in the '70's and we now think of as a chorus pedal. It's actually an unique multi-stage phaser, you can read the technical explanation here...
Real rotating speakers have a whole host of things going on. There is a crossover to seperate the highs and send them to a rotating horn and the lows to a speaker pointed at a rotating baffle. What you get is tremolo, phase shifting, the doppler effect and an immersive effect of sound waves being beamed all around the room before being bounced back at the listner from every angle and varying delay times (sort of direction reverb). Most of it's effect is lost on recording.