So right now it does not appear to be an easy way to monitor the other members within a Juniper Virtual Chassis, this method works with a secondary device that is a routing engine, in most cases your core switches or edge switches. This method does not work for monitoring members within a VC that is not a routing engine, example your other IDF switches if you have more than 2. In NPM when you list resource on a Juniper switch you can see the Virtual Chassis Port (VCP), enable monitoring on these ports and create and up/down alert on the interface, If the switch is down or missing a cable you will be notified. I will be testing this on a staging setup and will update my post if it was 100% successful.