Hello. The idea to have Tapatalk for the forum had been discussed before, but unfortunately due to some reasons we decided not to install it. You can read the discussions at this page.
Well, the reasons have never been made public. Tapatalk is pretty bad product considering the money they've earned by now but using Android it's unfortunately the easiest access to many forums.
If you don't want to install the plugin could you at least add a mobile theme? For non-profit vbulletin forums there are some nice ones for $25 on themeforest.com , maybe you'll find even a free one elsewhere.
If both the plugin and theme can't be installed due to other modifications you could create a theme yourself or ask the community to help.