I have four 6504-E switching in 2 VSS setups. Initially both VSS's were configured with only one VSL connection in the supervisor. Both VSS's had issues with the VSL going down at random intervals (sometimes hours, sometimes days), and the active switch chassis rebooting. I added a second VSL on each pair hoping to solve the issue. A week later both links went down at the exact same time and the active switch rebooted.
I'm running SXH3A on all for switches, I had the same issue with SXH2A.
Initially the active switch would reboot every 2-3 weeks or so, since I upgraded to SXH3a it's 2-3 times a week.