Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
Having looked at the specifications for both devices, I find that they are both NMEA0183 compatible. Why not just connect the NMEA 0183 output from the Vesper transponder to the NMEA 0183 port on the GPSMap ?
Is it the case, that all of the ports (on either or both device) are occupied already by other connections?
A little more information please: What cable from the Vesper transponder are you using?
Or, what cable from the Garmin are you speaking of? an internal cable? or something else?
That standard power-data cable that which one of the devices came with?