Reason 9.5 and later supports plugins in the VST2 (2.4) format. The VSTs also have to be 64-bit.
Note that VST3 plugins are not supported, and neither are 32-bit VST plugins. Also, VSTs that output MIDI are not supported in Reason.
Multitimbral VSTs (that can receive on separate MIDI channels) are not supported either. You will then have to use one instance of the VST for each sound/channel.