[trunk_public] Update vi_VN, thanks to : dummy

git-svn-id: http://piwigo.org/svn/trunk@23876 68402e56-0260-453c-a942-63ccdbb3a9ee
- on picture.php do not set the prefetch link if already set by a plugin
- picture.tpl set the derivative size checkbox id to the correct name

git-svn-id: http://piwigo.org/svn/trunk@23882 68402e56-0260-453c-a942-63ccdbb3a9ee