What do I need to create custom blocks and in particular the VIX part?
You only need the official LEGO MINDSTORMS EV3 software.
LEGO provides a "EV3 Block Developer Kit" on the MINDSTORMS downloads page. This contains some minimal instructions on creating blocks. It also contains an older version of the EV3 software.
However, I have found that any version of the MINDSTORMS software can be used in "developer mode" by creating a file named
MindstormsEV3.ini with the following contents:
<?xml version="1.0" encoding="utf-8" ?> <Tokens DeveloperMode="True" />
Save this file in the directory where the software is installed, e.g.
C:\Program Files (x86)\LEGO Software\LEGO MINDSTORMS EV3 Home Edition\, then start the software.