In the past, we attempted to develop for vB5 but unfortunately they never fully resolved all of the issues we reported back in 2012.
Essentially, the vB5 products are as-is and have been for several years, which is why there's a big red notice saying that feature upgrades are impossible due to design decisions made by Internet Brands.
If you need further evidence, look at this forum:
https://www.vbulletin.org/forum/forumdisplay.php?f=198
Look at the number of vB5 mods. In the
SEVEN YEARS since vBulletin 5 has been released, a grand total of
94 mods have been released. There are more than 94 XenForo mods compatible with version 2.1, which isn't even released yet.
It's quite obvious that no third party developer is actively developing for vB5.
In short: vB5 is not a platform suitable for 3rd party addons, and is not suitable for running in a production environment if you are interested in customising your site.