Please check that you have setup both the AIS and the NN3D port up for 38400 baud rate.
Since 2008 of the NN3D release, I have never heard of anyone not having standing targets. It could be you have the slower 4800 baud rate and the data is being limited by bandwidth.
If you are setup for 38,400; please open an official support case with your Furuno distributor. If you are a Furuno USA customer, that would be on our web site under SUPPORT and then on the left, select ASK FURUNO A QUESTION. Please provide photos of your AIS setup page and the raw NMEA 0183 (Port Monitor) screen (in the setup wizard) for the port you have the Vesper XB8000 connected. There could be some issue with the Vesper XB8000 NMEA 0183 sentence structure that isn't effecting your NMEA 2000, Wifi or USB outputs of the AIS. As you said, even you never noticed the problem when using the Standard Horizon unit. At this point, I would guess it is related to your baud rate or the NMEA data coming from the Vesper.