What system are you using? What you have done will work provided
that you are on Mac or Linux. If you are on Windows, you need
to swap the single and double quotes in the -if expression.
Also, you need to specify a filename, but hopefully that is obvious.
One more note: The -if condition will pass if any keyword
contains the string "03_Vortag", but the matching tag will only be
deleted if it is exactly "03_Vortag". This probably isn't a concern,
but just to warn you that keywords like "003_Vortag" will match but
won't get deleted.