This can only be one of two issues that can cause this.
1. You have input incorrect database information into the user's database spot of the classifieds config-inc.php file to connect and find the vb3 tables.
2. You have not input the correct table prefix for vb3 if you have any when you installed the application.
At any point you need to verify the settings table of classifieds and ensure if your running a vbulletin table prefix you enter it on settings 83 of the classifieds settings table. Now if you entered the same database info on install config as classifieds and you installed them in separate databases then you need to edit the config-inc.php file and place the proper info to access vbulletin database.
You can look at your vbulletin config.php file which is included in the includes directory of your vb
install to find out both database settings and prefix