Plaits has an in built VCA. If you are planning on using and external VCA make sure the one built in plaits is fully open then do what @eexee and @Lugia are suggesting.
I suggested you use an lfo as this way you don't have to worry about triggering it, if you plan on using an envelope you are going to also have to also learn how to to trigger it, or send a gate signal to the envelope.