I don't have the Vista Hcx, but the edge 605. I'm hoping that they're similar in this respect.
I have the mapsource gmapsupp.img for the entire UK on the internal memory. I have munkymaps gmapsupp.img on the microSD card. Both load at the same time and display, I can turn either on or off on the setup menu.
On my last holiday abroad I went through many countries, and Andy showed me the cloudmade maps (nowhere near as good, but worldwide OSM coverage). I put ALL of these on the microSD card as countryname.IMG. All I needed to do to get them working was fire up the eeePC, plug in the Garmin, rename the gmapsupp.img back to countryname1.img and countryname2.img to gmapsupp.img. Only took seconds as there was no data transfer required.
(I backed up the .tcx history files at the same time in case the GPS got lost/stolen so that I knew where I'd been on previous days).