Alright, so I added the "and enabled=1" to the sql_select statement. I tested sending email from a remote client. With a zero in the enabled field, the user cannot send email. With a one in the enabled field, email is delivered just fine.
As I see it, that is two out of three as I don't have Postfix actually configured to use the enabled field yet.
So I tried adding just the additional line at the bottom of mysql-virtual_mailboxes.cf:
additional_conditions = and enabled = 1
I sent an email to the account while it was disabled and Postfix still delivered it.
When I appended "and enabled = 1" to the query line instead and sent an email, I received an undeliverable message back saying the user did not exist. So that worked.
But why didn't the first way work? I have seen that syntax in many other tutorials.