Well the expensive answer is - and this is what we are doing now in our company - put another NIC in each server if there isn't a free one already and put new switches top of the rack exclusively for backup. i.e. it's not a VLAN it's a whole new network just for backup so you can effectively backup 24hours a day and no backup traffic is going over production links/interfaces.
We use juniper switches because they have a 6m uplink cable to stack the switches which is far superior to Cisco's uplink cable (in length I mean and of course we don't want to put a switch in every rack...there are over 50).
The juniper switches hang off a couple of Cisco 4500 and we reutilized our old PIX525 for security and filtering to leave nothing to chance
Bingo. Backup LAN completely isolated (well the backup servers, TSM in our case, have an interface in the backup LAN and production networks hence the firewalls).
Great solution: YES.
Expensive: YES.
Needed: YES in our case because we are hadling many servers, a lot of traffic and our backup window was full already. Plus the backup traffic was saturating networking links and monitoring at night was a nightmare. Red alarms everywhere until the backups finished.
Maybe that was a bit more than you wanted...or not enough, but I hope it helps.
Ian
