Virtuemart 3获得customfield params

i want get a spesific customfield params on category page(category/default.php),but until now i still can't find a solution, i tried these method but all not work

method 1

$plugin = JPluginHelper::getPlugin('vmcustom', 'mypluginname');
$params = new JRegistry($plugin->params);
echo $params->get('param_name','default_value');

method 2

$plugin = &JPluginHelper::getPlugin('vmcustom', 'mypluginname');
$pluginparams = new JRegistry($plugin->params);
$paramvalue = $pluginparams->get('myparameter');

i think maybe these not working,because VM3 version use <vmconfig>,don't know is it right,so how can i get a spesific customfield params on category page?

Joomla 3.4.8 / Virtuemart 3.0.12 / Php 5.4

many thanks!