Does anyone know what format DEM files are used with VistaPro?

I have an old copy ( v4.0) of Vista Pro, and it has a whole CD of DEM files, but Vue refuses to import them. What I have to do is import them into Bryce (5.5), then export as either obj or "copy" the image in the Bryce Terrain Editor, and paste it into a photo program before importing into Vue.

That makes for a long process, and it would be much easier if Vue could read these files!