How to query custom table

[cowbert]

I created a custom mysql table using a separate prefix and I wrote a plugin to read from it in vB, but when I do:

PHP Code:

$result = $vbulletin->db->query_first_slave("
   SELECT some_field
   FROM my_custom_table
   WHERE some_field = 'some value'"); 


It returns error:

Invalid SQL:
  
   SELECT some_field
   FROM my_custom_table
   WHERE some_field = 'some value';
  
MySQL Error   : Table 'vb.my_custom_table' doesn't exist

Even though when I am in mysql CLI as the vb database user and:

mysql> select * from my_custom_table;
+------------+
| some_field |
+------------+
| foo        |
+------------+
1 row in set (0.00 sec)

The sql call fails via the "Execute a SQL Query" via admincp as well. How do I get my plugin to read my custom table? Do I have to do an end run around and create a 2nd new database connection outside of using the vB objects?