On install rerun make sure you do several things here.
1. On config screen make sure to put in your correct vb3 license number and in user database portion put your vb
2. On step 4 select vb3 from the dropdown and enter the mysql table prefix and cookie prefix usually bb where asked.
You then go through and complete the install and you should be fine to login without issue. I suggest also when your all done that both cookies on photopost and vb3 be set like this
cookie path /
cookie domain .domain.com
cookie prefix bb