Could the JSON document type defined in the VSI be causing an issue?
Since you are referencing a DevTest property here the content is not technically a JSON document at this point. Also, you will not see the actual property value in the VSI - just the property key.
Try changing the type to General, Text and see if the bad string goes away.
If you wanted, you could add a Scripted Assertion in the VSM's VSI Selection step to dump your actual response value into the log.
import com.itko.lisa.vse.stateful.model.TransientResponse;
import java.util.List;
List list = testExec.getStateObject("lisa_vse_response");
TransientResponse tr = list.get(0);
theBody = tr.getBodyAsString();
// Since the body contains a value of "{{MyExternalDataFile}}", use parseInState to show the 'real' content
_logger.info("My Response is: \n{}", testExec.parseInState(theBody);
// set the Assertion to fail if FALSE so that returning true moves on to the next step.
return true;
If you are running in VSE, the log output is written to vse.log. If you are running the service in ITR mode, the log is output into Workstation.log (i.e., c:\users\<yourID>\lisatmp_x.x\workstation.log)