Checkthegate - Just trying to clarify - are you using the heightmap in the "Displacement" channel, or ar you using it as a height map in the Terrain Editor (fundamental difference - if you're not!)?

And I dunno about anyone else here, but I have noticed stepping whe I use 8-bit greyscale images, and it's nice and smooth when I use 16-bit. To overcome the 8-bit stepping, I simply click ONCE on a smoothing erosion effect, then coninue to sculpt to my heart's content.

Vue only handles what can be carved out vertically - if you have an overhang, or natural bridge type of formation, you'll have to do it in ZBrush (or your modelling app of choice - I noticed you also mentioned Maya above).

You can save out of ZBrush as an OBJ and apply ecosystems, plant vegetation, just like a Vue terrain, so think of using it in combination with your other apps in a final assembly and rendering situation so you get outdoor atmosphere lighting. You can even replace the sky with any image you choose if you custom paint your own, or take your own photographic skies like I do.

Hope this helps-
-Lew ;-)