From the VK-8M manual...

❍
General Purpose Controller 5 (Controller number 80) (Rotary Speed)
Status 2nd byte 3rd byte
BnH 50H vvH
n = MIDI channel number: 0H - FH (ch.1 - ch.16)
vv = control value: 00H - 7FH (0 - 127)
* When MIDI IN FUNCTION is SINGLE KEYBOARD MODE (p. 20), this is received on
the channel specified for CONTROL MIDI CH. (p. 46) (factory setting: channel 1).
* Not received when MIDI GENERAL CONTROLLERS SWITCH (p. 45) is OFF.
❍
General Purpose Controller 6 (Controller number 81) (Rotary Brake)
Status 2nd byte 3rd byte
BnH 51H vvH
n = MIDI channel number: 0H - FH (ch.1 - ch.16)
vv = control value: 00H - 7FH (0 - 127) 0-63 = SPIN, 64-127 = BRAKE
* When MIDI IN FUNCTION is SINGLE KEYBOARD MODE (p. 20), this is received on
the channel specified for CONTROL MIDI CH. (p. 46) (factory setting: channel 1).
* Not received when MIDI GENERAL CONTROLLERS SWITCH (p. 45) is OFF.

It appears you need to send MIDI controller cc#80 to get F/S Leslie control