Don't know have Vista - but on the system spec on Pixologic it says:
ZBRUSH 3.1 - WINDOWS
Recommended:
OS: Windows XP/
Vista
CPU: Fast P4 or newer (or equivalent such as AMD) with optional multithreading or hyperthreading capabilities
RAM: 1024MB (2048MB for working with multi-million-poly meshes)
Monitor: 1280x1024 monitor resolution or higher (32 bits)
Minimum System Requirements
OS: Windows 2000
CPU: PIII
RAM: 512MB (1024 MB recommended)**
Monitor: 1024x768 monitor resolution (32 bits)
Also there is a trial version that you can try on your PC before buying.
http://www.pixologic.com/zbrush/trial/