Starting with OpenMPT 1.20.02.01
(that's sadly one version after the official release from last Saturday!
), it is possible to use the standard Hxx/Kxx or 4xx/6xx effects to send vibrato to VSTis. How does this work and what do you have to keep in mind?
Vibrato is implemented through the pitch wheel, so first off the VSTi that you want to apply vibrato on must support pitch wheel messages. Many plugins do that, and if you can use Exx/Fxx effects with that plugin, it will also support Hxx.
The vibrato scale has been chosen so that it is absolutely identical
to sample-based vibrato if the pitch wheel depth is set to +/- 2 semitones. This is the default setting for many synths. Many synths allow you to change the pitch wheel depth either through their GUI, a VST parameter and / or through MIDI CCs. The default MIDI CC sequence used to control the pitch wheel depth is "Bc6400 Bc6500 Bc06z" - you can set up a MIDI macro for this and then use Z00... Z7F to configure the pitch wheel depth (the parameter should translate directly to semintones, so Z02 would set a pitch wheel depth of +/- 2 semitones).