I'm using the wip/network-test-server from Gerrit (albeit rebased onto a recent 5.6) to run the Vagrant test server (cd tst_qnetworkproxyfactory && vagrant up, then fix local /etc/hosts to point qt-test-server.qt-test-net at 192.168.33.10).
From a separate checkout of pristine current 5.6, I have done a shadow build of qt5 and, in its qtbase/tests/auto/network/access/qftp/ directory, I run

LANG=en.UTF-8 DISPLAY=:1 LD_LIBRARY_PATH=$SHADOW/qt5/qtbase/lib:$LD_LIBRARY_PATH QT_PLUGIN_PATH=$SHADOW/qt5/qtbase/plugins ./tst_qftp activeMode

See attached tst_qftp.log for boring details: after five minutes, the test system's WatchDog kicks in and kills the test.
When I run with the public network test server (qt-test-server.qt-test-net is 10.212.2.216), the tests all pass; the command above takes about a second to run.

It would seem the Vagrant server's support for ftp/socks is not satisfactory.