Ah, OK, I see.
If the VSI contains XML, perhaps you could add a Scriptable DPH to the response side to convert the XML back into JSON format. Based on the json-20140107.jar located in LISA_HOME\lib\shared, the code would use something like the following (pardon typos) :
import org.json.JSONObject;
import org.json.XML;
int INDENT_FACTOR = 4;
try {
JSONObject xmlJSONObj = XML.toJSONObject( lisa_vse_response.get(0).getBodyAsString() );
String jsonPrettyPrintString = xmlJSONObj.toString( INDENT_FACTOR );
lisa_vse_response.get(0).setBodyText( jsonPrettyPrintString );
} catch (JSONException je) {
// leave response as XML and write exception info into the VSE log file
_logger.info( "<<<" );
_logger.info( "<<< Error Converting XML to JSON. Msg is:\r\n{}", je.toString() );
_logger.info( "<<<");
}
Additionally, the code may need to cater for the VSI's HTML-based Stateful / Stateless Service Image Not Found responses. An SI Not Found response, might throw an exception since it is not XML-based unless you changed it in the VSI.