In my post above I stated this below which stated exactly where the prefix was found.
Your cookie prefix is in your vb config file and the default is bb
In our Photopost documentation cookie settings are thoroughly covered for vb3 users as lots of people make this mistake. Most times alot of issues would be resolved with the use of the search engine. However we know the world is not perfect and yes there are many threads on the same mistakes users make over and over again. vB3 Integration FAQ