Yeah I could do this on a staging env. Ill try. Is there a list of Invision tables floating around anywhere? So to be sure.
1. Create a new DB with a new prefix and new user and a new DB name
2. Copy my existing WP DB to this new DB
3. Change Invision config file so it uses this new DB (Prefix, password etc)
4. Go through this new DB and delete out all the WordPress tables
Sound about right?