Hi unca D,
I've never used outboard sound modules with Reaper but I'm assuming the hookup would be something like the picture below.
This assumes both the Keyboard and Tascam Interface can transmit it's midi in and out through the USB.
On the Midi Input track you'll need select
Input: MIDI and direct it to the Tascam Interface Midi Input and set it to what ever channel you are using. You can probably set it to All Channels but you will have to send the appropriate midi channel to the Tascam Interface (the one you're using in your JV1080).
Also on the
I/O of the Midi Track you'll have to select
MIDI Hardware Output (upper right corner) and direct it to the Tascam Interface. Just below that, select the midi channel you want to send the JV1080.
On the Audio Track for the JV1080, I'd select the Stereo Input directed to the Tascam Inputs where you hooked the JV1080 to the Tascam. Either that or use two input tracks for L & R. I'm assuming the JV1080 is stereo.
If I've got this all wrong, maybe DarkStar will have a better way.
EDIT: Heh heh, I see I spelled module wrong but oh well.