MSExchangeteam has blogged how to troubleshoot VSS backups.
There have been some blogs on VSS backups in the past. Over last few months, we have worked on several cases where troubleshooting VSS was required. Seeing that the documentation around how to troubleshoot this is somewhat lacking, we decided to put together this blog post. This post will give you VSS troubleshooting tips using publicly available tools.
Read more at source: http://msexchangeteam.com/archive/2008/08/25/449684.aspx