Wrong number of arguments
vc-find-root must be invoked with two arguments: (vc-find-root FILE WITNESS).
To get vc root use vc-root-dir instead:
vc-root-dir is an autoloaded compiled Lisp function in vc.el.
Return the root directory for the current VC tree.
Return nil if the root directory cannot be identified.
And if you need the vc root of ...
As is usually the case right after one posts a question, you'll find an answer elsewhere.
After looking in the sources (lisp/vc/vs-svn.el) we learn that unless under darwin svn will be given the --non-interactive option. This seems to be the problem when subversion is using the passtype gpg-agent. Changing it to --force-interactive works (deling it does not)...