Then you must look in your tables. The query is correct and works. You must of course also replace this part ' . (int)$_vendorId . ' against your vendorId, usually 1.
lets debug the vendorId.
Do not use this nonsense here
print 'Debug Line '.__LINE__.' $_vendorCurrencies <pre>'; print_r (self::$_vendorCurrencies[$_vendorId]); print "</pre><br />\n";
just use instead
vmdebug('my vendor id and currencies',$_vendorId, self::$_vendorCurrencies);
and enable the vmdebug. Maybe your vendorId is zero, or some unexpected number.[/code]