Can't set orderstatus on VRA(Vendor Return Authorization)

I set a workflow with a custom action called Set Field, which has 2 parameters : fieldId and value. Workflow action script like this:

    var scriptObj = runtime.getCurrentScript();
        var myField = scriptObj.getParameter({name: 'custscript_field_set'});
        var myValue = scriptObj.getParameter({name: 'custscript_value_set'});
        ……
        myRecord.setValue({
                fieldId  :  myField,
                value   :  myValue,
        });
        myRecord.save();

I use the custom action to set transaction's orderstatus with parameters: orderstatus,B (make transaction approved).
And it works on Sales Order and Return Authorization, but can't change status on VRA(Vendor Return Authorization) on any trigger. it makes me almost angry. Hope someone could help me!