I recommend against creating these “placeholder” secret values. There is no need to do so. Vault KV stores work a bit like Amazon S3 buckets - there is no such thing as a directory physically in storage, directories are just implied, based on prefixes ending with a slash character, in the full path to “files” stored.
Provided your Vault ACL policies allow it, the application will be able to write to apps/dev/app1/foo/bar/baz directly, whether or not these placeholder secrets exist.
Furthermore, if you do create these placeholder secrets, and the app creates the previously suggested apps/dev/app1/foo/bar/baz, and then you do vault list apps/dev/, then the result will be:
i.e. it is permitted - but sometimes confusing! - for a “file” and a “directory” to exist at the same path, in a Vault KV store (or indeed S3 bucket).