With the scripts you can "turn on" vibrato at any time while you hold the note.
Some notes on using the scripts:
Load Flute Solo NV.nki, insert the 1st script.
Load Flute Solo V.nki to the second slot, assign it to the same MIDI channel, insert the 2nd script.
Now you can "turn on" vibrato by inserting CC74 (of any value > 0) in your sequencer. At time of insertion Flute Non Vibrato starts fading out and Flute Vibrato starts fading in. I wrote the scripts for controller 74. You can change it to anything you like. So far it works for two instruments. I'm sure it's possible to merge them into one. I don't have a host to post actual .nkp files. So, here are two scripts for GPO flutes (sorry, they look ugly - somehow I couldn't use Indent):

{Script "Fade out" for GPO Flute Solo NV.nki}
on init
declare $start_fading
end on

on note
wait(10) {doesn't work without wait}
$start_fading := 0
while ($NOTE_HELD = 1)
wait(100)
if ($start_fading = 1)
fade_out($EVENT_ID,200000,1) {1 means stop sounding after fade out}
end if
end while
end on

on controller
if (%CC[74] > 0)
$start_fading := 1
end if
end on
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{Script "Fade in" for GPO Flute Solo V.nki}
on init
declare $start_fading
end on

on note
wait(10) {doesn't work without wait}
$start_fading := 0
wait(100)
fade_out($EVENT_ID, 10,0)
while ($NOTE_HELD = 1)
wait(100)
if ($start_fading = 1)
fade_in($EVENT_ID,200000)
end if
end while
end on

on controller
if (%CC[74] > 0)
$start_fading := 1
end if