Larryo,

NMEA-0183 was designed for point to point connections between two devices.  So, if you had a DSC VHF radio that didn't have built-in GPS you could use the 0183 output of your MFD or a dedicated GPS unit to feed it location information.  It sounds like your VHF radios are basically self-sufficient and since you have an AIS transceiver likely already on your NMEA-2000 network there isn't a reason to connect them to the NMEA-2000 network unless there was further integration you're after (like DSC station to station calling via the MFD).  

An external GPS is likley to be more accurate than a patch style GPS antenna burried in the radio at the lower helm.  I'm not sure which radio you have and how it selects among GPS sources but one potential downside to using your MFD's GPS output would be the need to have the MFD powered on for the radio to get location information.  You could solve that by using an NMEA-2000 -> NMEA-0183 converter to feed position information direct off your N2K network assuming there's a source on the network (which is extremely likely with a 2017 era electronics package).

Ben

Ben,

Thanks much. I have an Evo 3 at the lower and Evo 2 and 3 at the upper. There are times when I'd rather not turn the upper electronics on for a short trip, but getting GPS from the lower unit without an antenna is hard, so would like to tie the antenna into the lower with a splitter. Same with the lower VHF... hard to get a good GPS signal from the unit, so tie that also to the splitter. Does this make sense?