This portion of the forum is intended for reporting the discovery of possible software bugs. Questions on how to use FileMaker Pro should be posted to the FM Pro Forum. (And had you done so, I would have seen and responded to your question hours sooner....)
There are a lot of unaswered questions about what you need to do here.
First, a POS system usually has many more tables with relationships similar to this:
Each record in Clients records data on a given client (many POS systems do not have this table). each Record in Inovices is a specific Sales Transaction. LineItems lists the individual items purchased and their quantities on a given invoice and also links to Products for pricing, description type data.
Do you have this kind of system set up?
Does a "VIP code" idenfity a specific client or just a specific discount?
The only real difference this makes is in what table you'd store the barcodes and any relevant information about each such code.
There are a number of threads that discuss how to set up a barcode, magnetic strip or smartcard scanner to work with FileMaker. As long as your scanner can operate in "keyboard emulation mode" (almost all can), you can use the "search the forum" box at the top of this screen to look for posts about "barcode scanner" to pull up a number of threads that discuss how to design layouts and scripts to support barcode scanning.