The VST 3 SDK comes with a helper class which wraps one VST 3 Audio Processor and Edit Controller to a VST 2.x Plug-in.
You just need to add public.sdk/source/vst/vst2wrapper/vst2wrapper.sdk.cpp to your project and add the following code somewhere in your sources:
#include "public.sdk/source/vst/vst2wrapper/vst2wrapper.h" //------------------------------------------------------------------------ ::AudioEffect* createEffectInstance (audioMasterCallback audioMaster) { return Steinberg::Vst::Vst2Wrapper::create (GetPluginFactory (), kAudioProcessorCID, kVst2UniqueID, audioMaster); }