You can choose for some solution like EMC Networker which are cluster aware and can take online backup of your databases.

Actually I can't backup virtual MS SQL Server using NW with MS SQL module. I can backup only physical nodes instead.

Why so? I believe NW should backup the virtual node as well. Any specific problem you are facing.

Hi,

for backup you need to know the "Network name" resource value for virtual SQL server (check your Cluster Administrator), define that Network name as a NetWorker client, set the Backup command for that NetWorker client to nsrsqlsv -a , know any named instances of virtual SQL server, define them in "Save Sets" field for this new NetWorker Client and you're on your way...

best regards
Mladen