When I open the drum set (both auto and manual) in FL Studio, I can KS the snare on and off but in Sibelius the same keys do nothing.

Sibelius won't likely act like a normal sequencer environment. Usually things like keyswitches need to be triggered via dictionary text or something similar. It would depend on what sound set you're using and what 'instrument' is assigned to that particular staff.

Most people using VDL in Sibelius use a template from The Write Score to do all that sort of KS manipulation which would then be done by simply typing the words "snares off" in the score.
http://www.thewritescore.com