One really nice thing that reason and nuendo have is called REWIRE. What you do is open nuendo, and select reason and a VST or there may be a selection for rewire on its own. Once you select that and get the channels set up, open reason and load the song. You should now be able to control the start aned stop of reason via the transport on nuendo and you can record into nuendo while playing reason. Another opetion is to export you song from reason as an audio file and load that into a track or two of nuendo. that way, you have the actualy audio file in nuendo so there may be less latency.