Review these articles:
http://blogs.msdn.com/sqlserverfaq/archive/2009/04/28/is-sqlvdi-dll-functioning-properly.aspx
http://support.microsoft.com/default.aspx?scid=kb;EN-US;934396
Are you backing up across the network? If so, you need to make sure your network is capable of supporting SQL Server backups across the network. If not, you will get these kinds of errors also.
Jeffrey Williams
“We are all faced with a series of great opportunities brilliantly disguised as impossible situations.”
― Charles R. Swindoll
How to post questions to get better answers faster
Managing Transaction Logs