Use method clean()
from class Phoenix_VarnishCache_Model_Control
for flush varnich cache, more info you can see in this class.
/**
* Flush all varnish cache
*
* @var cache Phoenix_VarnishCache_Model_Control
*/
try {
$cache = Mage::getSingleton('varnishcache/control');
if ($cache) {
$cache->clean(Mage::getBaseUrl(Mage_Core_Model_Store::URL_TYPE_WEB));
}
} catch (Exception $exc) {
Mage::log($exc->getTraceAsString(), null, __METHOD__.'__ERR.log', true);
}