OT, and not really an answer to your question, but this is important
(a former employer of mine learned the hard way):
some versions of VSS (maybe all?) don't check disk space
before committing, and don't behave gracefully when it runs
out.
This problem exists until at least version 6. At a former company we
also found out about this problem the hard way. At the time (this was a
few years ago) Microsoft would not say if/when this problem would be
addressed.
Fortunately we had backups and only lost 1 day of work.