Vcarve is not so precise because the software converts 3D objects into heightmaps which then get used to generated toolpaths. Meaning the size of your pixel determines how precise your 3D milling will be and Vcarve has a limit for the amount of these pixels. So your milling job can be precise up to 0.5 mm, everything better than that is luck
This is what the edge of a sphere looks like in vcarve:
