When are privileges listed in \l and when not?
The backslash commands in psql are shortcuts for a query or queries that look through the system catalogs. The \l command looks at information in pg_catalog.pg_database, specifically, this query:

SELECT d.datname as "Name",
   pg_catalog.pg_get_userbyid(d.datdba) as "Owner",
   pg_catalog.pg_encoding_to_char(d.encoding) as "Encoding",
   d.datcollate as "Collate",
   d.datctype as "Ctype",
   pg_catalog.array_to_string(d.datacl, E'\n') AS "Access privileges"
FROM pg_catalog.pg_database d
ORDER BY 1;


You can make psql show what it is using for the backslash commands by passing the -E flag to it when you invoke it on the command line.

If the permissions on a database or other object are the defaults that PostgreSQL creates them with, the *acl column will be NULL. If you change the defaults, as you have, the ACL column will be populated with information related to the GRANT and/or REVOKE statements you have ran.

You can see the permissions/ACLs specifically via either \z or \dp

If you read further here:

http://www.postgresql.org/docs/9.4/static/sql-grant.html

If you scroll down, (or search for the word psql), you can look at the table that shows you how to interpret the ACLs that you see with \l or in an ACL column.

For example: 

=Tc/vagrant

means that PUBLIC (the implicit role that contains all roles) has permissions to create temporary tables T and connect c, because the ACL line =xxxxx denotes permissions applied to PUBLIC, while rolname=xxxx applies to that specific role.

This presentation from Dalibo should also help clarify this further: Managing Rights in PostgreSQL

Hope that helps. =)