Vai uses an Eventide harmonizer. I don't think Malmsteen uses a harmonizer, at least none that I've seen over the years.
Not even on the same page as chorus. If you're an actual fan of Steve Vai, check out the song Ballerina 12/24. He used an Eventide H3000 on that song. For an example of what chorus sounds like, go to musiciansfriend and look for a chorus pedal. Most of them have soundclips you can listen to.