IMO you do need tblLoggedIn. It would contain the userID from tblUsers (e.g. 22 - the autonumber value) and to make that easier to decipher, the Fname and Lname fields plus whatever else you need. A checkbox is minimal and doesn't tell you much. If you want an ongoing log use Now() for the date/time as DateTimeIn and DateTimeOut fields. When a user logs in your code would append a record and when they log out, update DateTimeOut. The drawback is that abnormal db shutdown by the user can leave DateTimeOut as Null so if you intend to control how many sessions a user can create you'll have that issue to work around. I don't think a log file is necessary if you have this in a table, but that's just my opinion. I might use a log file to flag general issues though. For that, research CreateObject("Scripting.FileSystemObject"). You can even give your file a .log extension and Notepad will open that.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.