I've never done this before myself, since Visual Assist is pretty darn fast at indexing everything. But you can try going too "VAssistX->Visual Assist X Options->Performance" and uncheck, "Parse all files when opening a project".

It rebuilds the database anytime I load any solution, same name or not. Not really sure how they could do anything else, unless they cached the time stamps and used that to determine if anything changed since it was last open. Even then, source control solutions like Perforce have the option to keep the timestamp from the server, which means I could sync to older revisions of files and they wouldn't be parsed.

It takes < 10 seconds for it to parse when I load solutions, so I generally just ignore it.