Good Afternoon,
Is the VHF outputting AIS data at 38400 baud rate and expecting to receive GPS data at 4800 baud rate?
If so then this could be your issue, the NGX-1 is only capable of outputting and receiving on a single baud rate. so either 4800 or 38400.
If the VHF has the ability to change the baud rate it receives GPS data on to 38400, then this should solve your issue.
If not, then I am afraid you will likely require an additional converter to handle the data on a separate baud rate.