Hi Julian,
Yes, the log event "MPLS.L2VPN.ChgPWState" will increment when the VPLS/VPWS state changes between UP/Down.
You can use the command below to check the counters:
show log counters MPLS.L2VPN.ChgPWState
To add the event to the "DefaultFilter" log you can use the command below:
configure log filter "DefaultFilter" add events MPLS.L2VPN.ChgPWState