This command display the internal path across the EX-4200 PFE chips.
I obtain a result different that the EX-4200 diagram displayed in the O'reilly book "JunOS enterprise Switching" regarding the "Uplink Module".
Here are my results (simplified to a 3-members stack):
The Juniper use an internal routing-protocol for using shortest path between ports (less PFE hop count).
This diagram depends of how you build your stack too: If you use another way for plug your stack-cables, the result will be different.
Here is how to display the path when going from port ge-0/0/0 to ge-2/0/0
user@ex-stack> show virtual-chassis vc-path source-interface ge-0/0/0 destination-interface ge-2/0/0
vc-path from ge-0/0/0 to ge-2/0/0
Hop Member PFE-Device Interface
0 0 1 ge-0/0/0
1 1 4 vcp-1
2 1 5 internal-1/25
3 1 3 internal-2/24
4 2 6 vcp-0
5 2 8 internal-0/24
6 2 7 ge-2/0/0
This means that if only one of the 3 PFE on one of your stack member have a problem, it's all flow that cross this member that will have problem.
You can check internal-PFE communication error with this command:
show virtual-chassis vc-port statistics extensive | match errors