Does anyone know can Cisco ACS be used as syslog server? For collecting logs from cisco devices?
Cisco ACS can't be used as syslog server, but it could be used as AAA server which is include Accounting:
if you are looking to use ACS as accounting server for management account which is include logging for the commands and changes done by any ASA user you have to define the ACS as tacacs server for the ASA you could find more info on this link: ASA 8.3: TACACS Authentication using ACS 5.X
or if you want to use it for VPN accounting, you should enable the VPN authentication and authorization on the ASA to use ACS as radius server and in this case you can retrieve the VPN users sessions details from the ACS logs
you could find more info on this link: Radius Authorization (ACS 5.x) for VPN Access