if you're using an instrument besides zyn or a vsti, couldn't you just make use of the note detuning feature and make vibrato that way?
my name is aden (he/him), and i'm a devout user of libre software! i started out making music with lmms and rosegarden, but then switched to sunvox for a long while. now, i'm back to lmms, and i couldn't be more happy! i built the latest github master (which i saw now has native linux vst support) a...
have you tried using the pitch wheel in the instrument window?

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you may have to manually set the pitch bend range in whatever plugin you're using