To move your apache vhost directory in plesk do the following

cd /usr/local/psa/bin
./transvhosts.pl --source-dir /my/old/directory --dest-dir /my/new/directory --correct-scripts

The new directory must exist even though the script help says it will create missing directories it did not work for me. The correct script option will change all the vhost configs for you. Watch out for trailing slashes or you will have incorrect paths in your vhost configs. Another thing i noticed some of our vhosts had double slashes in there path names to correct this issue

/usr/local/psa/admin/sbin/websrvmng -a -v