It appears that the Vault command-line syntax changed between v1 and v2 and the -overwrite flag [1] is no longer supported for gets.
[1]
http://lists.sourcegear.com/pipermai...er/001918.html
We'll add it to our list to also support the v2 syntax in a future release. For now, as you already found, you can specify this flag in the additional options field. Thanks for the report.