How to run the REU-version:

1541 Ultimate II or Ultimate 64
  Press F2, select C64 and Cartridge Settings:
  - Set Cartridge to None
  - Set RAM Expansion Unit to Enabled
  - Set REU size to 2 MB
  - RUN/STOP to return to menu
  - Select file vn72.reu and then Load into REU
  - Select file vn72.prg and then Run

VICE (3.6.1 and above should be used)
 - Ensure existing cartridges are removed (File > Detach)
 - Settings > I/O Ext > RAM Expansion Module
 - Enable REU and select size 2 MB (2048KB)
 - Select the REU image (vn72.reu) and exit dialogue
 - Drag and drop the file vn72.prg into the emulator

VICE on any OS from the command line
  x64sc -reu -reusize 2048 -reuimage vn72.reu vn72.prg