The problem with Vector is that it is now almost 7 years out-of-date and it is not currently being maintained. At some point, there will be an unresolvable conflict or absence of support.
For instance, the old menu system now has only legacy support, which means that template authors have to switch over to the new system, because the old one will be discontinued. When this happens, users of templates that have not been updated will not be able to upgrade their dokuwiki. Also, dokuwiki makes use of PHP versions that allow for new PHP features, and so an old DokuWiki may run on older systems but not on newer ones and for that reason alone may not be updatable. Old DW's are then not able to take advantage of the continuous DW security fixes.
