The actual error means you have not put valid certificates into your truststore.
The problem is not with the tool, IMHO, the problem is with the developer not understanding the basic concepts about PKIX path building, which is essential if you want to use SSL.
You should be using this form of the call, which you can turn SSL off if you do not want to use it, by invoking FtpFileSystemConfigBuilder rather than SftpFileSystemConfigBuilder
Then again, if your wanting to perform RESTful calls exclusively, your better interface would be Jersey rather than VFS.