Welcome to the board, as you can tell I don't post much myself
Your first question about using one database or table to perform as a main login point so that your users do not have to re-register is possible. There is a few ways you can do this, you will want to make one of the scripts the main one with the main table. Vbulletin works great for this, or has in the past for me when I was working on a project for someone. You can use some joining statements to combine the tables of vbulletin and your dating script. Because the dating script will have some fields that no other table will have (yes you can add these fields to any table you wish).
This one is not all that tough but will take some time to get it all working correctly but to answer the question it can be done. If you look into one of the book and also look at how the code is structed you can figure out how to do some joins... of the databases, you just have to have some common fields that are the same. Username, a specific ID field or something to link them together.
As for your 2nd question, you might be able to get by, by using a credit card processing script, creating a simple database with admin controls and a user front, that can process and verify the card then pull the pin from the database and delete or move it to a "used pins" table for archiving.
Brad