I called into JTAC about this because the switches were behaving oddly after I set this up. The show virtual-chassis command indicated they were working fine, but other commands would cause the switches to freeze up for 5 minutes or so and say they couldn't communicate.
I wound up having one switching running 15.1X53-D50 and one running 15.1X53-D55 so we upgraded them to the same version.
After that, we cabled them and ran the command request virtual-chassis vc-port set pic-slot 1 port 3 on the switch we wanted to be the master, then ran the same command on the other. In my setup, I was connecting my DAC cable into port 3 of the 4x1/10 gb sfp module... hence pic-slot 1 port 3
show chassis hardware will show you what you pic-slots are
Other commands JTAC added were:
set virtual-chassis no-split-detection *** only in 2 switch virtual chassis setups
set system commit synchronize
set chassis redundancy graceful-switchover
set routing-options nonstop-routing
set protocols layer2-control nonstop-bridging