diff --git a/libs/ardour/plugin_insert.cc b/libs/ardour/plugin_insert.cc
index ee8016c6af..a1c2e4b165 100644
--- a/libs/ardour/plugin_insert.cc
+++ b/libs/ardour/plugin_insert.cc
@@ -2437,6 +2437,11 @@ PluginInsert::add_plugin (boost::shared_ptr<Plugin> plugin)
 			}
 		}
 	}
+
+	boost::shared_ptr<VSTPlugin> vst = boost::dynamic_pointer_cast<VSTPlugin> (plugin);
+	if (vst) {
+		vst->set_insert (this, _plugins.size ());
+	}
 	_plugins.push_back (plugin);
 }
 
diff --git a/libs/ardour/session_vst.cc b/libs/ardour/session_vst.cc
index c6978eddc7..0e16ac1ebe 100644
--- a/libs/ardour/session_vst.cc
+++ b/libs/ardour/session_vst.cc
@@ -26,6 +26,7 @@
 #include "ardour/debug.h"
 #include "ardour/session.h"
 #include "ardour/tempo.h"
+#include "ardour/plugin_insert.h"
 #include "ardour/windows_vst_plugin.h"
 #include "ardour/vestige/aeffectx.h"
 #include "ardour/vst_types.h"
@@ -124,11 +125,25 @@ intptr_t Session::vst_callback (
 		}
 		switch (value) {
 			case 0:
+				if (plug->plugin_insert ()) {
+					bool valid;
+					const ChanMapping& map (plug->plugin_insert ()->input_map (plug->plugin_number ()));
+					map.get (DataType::AUDIO, index, &valid);
+					return valid ? 0 : 1;
+				}
 				if (index < plug->plugin()->numInputs) {
 					return 0;
 				}
 				break;
 			case 1:
+#if 0 // investigate, the outputs *are* connected to scratch buffers
+				if (plug->plugin_insert ()) {
+					bool valid;
+					const ChanMapping& map (plug->plugin_insert ()->output_map (plug->plugin_number ()));
+					map.get (DataType::AUDIO, index, &valid);
+					return valid ? 0 : 1;
+				}
+#endif
 				if (index < plug->plugin()->numOutputs) {
 					return 0;
 				}