try to play around with midi cc commands (CC xxyy, wher xx is the command code, yy is the parameter). they say about 11 that: "11 Expression (MSB) Roland synths use it. Some synths use it for LFOs, some for crescendo/ decrescendo (loudness). Sometimes routed to keyboard aftertouch."
In FL studio you can decide which to send, CC or aftertouch, when I send aftertouch it controls the vibrato. Is it possible to send aftertouch command?