You'll have much better security if you use FileMaker's built in security rather than trying to create your own. You can use functions like Get ( AccountName ) to identify the user in your scripts.
I've made a duplicate file to try this out. The user needs to be able to select a LOGIN button on the home screen, which triggers the login script. Currently, the login scripts is :
^ Re-Login 
^ Go to Layout ["Vendors" (Vendors)]
^ Go to Record/Request/Page [No dialog; Get ( AccountName ) = Vendors::BoothID]
In the CREATE NEW USER script, the user name is set to the vendor's booth number, or (Vendors::BoothID)
Everything seems ok, but the script will not point the user to his/her record, it will direct the user to the first record in the found set, and not the record that adheres to the last line of the login script.
Go To Record/Request/Page
Is used to change the currrent record to be a different record in the current found set and you are specifying the record by it's position, not by any value in that record. You are basically saying: "go to the 1st record" or go to the 0 record (which doesn't exist). That's because your boolean expression, Get ( AccountName ) = Vendors::BoothID will either be true ( go to 1st record) or false ( go to the 0 record ) and it is evaluated that way before the Go To Record step is performed.
Instead of using go to record, try performing a find for that user's record or records.
Go to Layout ["Vendors" (Vendors)]
Enter Find Mode  //clear the pause check box
Set Field [Vendors::BoothID ; Get ( AccountName ) ]
Set ErrorCapture [on]
Perform Find 
This assumes that the account name is what is entered in BoothID--which may not be the best way to match vendors to their account names.
Thanks again Phil!
That seems to work very well.
As for the Account name being the boothID, that is a placeholder until I get a little deeper into this mess.
I am a one-man-band here so your help is much appreciated.
We have had great success using FileMaker so far for a variety of uses. Most of my posts on here are related to the back-end system I now use to coordinate live event details, but we have also used FileMaker successfully for trade-show registration and it has served well even with 90 registration stations and 35k+ attendees in que.
As soon as possible, consider using a serial number ID for the BoothID and then a table where current account names are matched to the ID numbers. This will be much more flexible and allow account names to be changed without losing connctions to your vendor records.