With MidiTransposer you can transpose your keyboard on the fly, simply pressing a button and a key.
Open it in your VST Host application, Connect the host midi input to the MidiTransposer's midi input and its midi output to the midi input of a Vst instrument.
Press and hold a button of your midi keyboard sending a CC message #110, set the desired transpose interval with reference to the C3 key (midi note number 60) pressing the corresponding (upper or lower) key, then release the button: the chosen transpose interval is shown in the MidiTransposer's GUI in terms of upper or lower (-) semitones.
In order to cancel the transposition (reset) simply press a button of your midi keyboard sending a CC message #111.
You can change the CC messages to be used for transposing and resetting the transposition through the relevant drop list in the MidiTransposer's GUI.
