Understand the difference between the mobile app, and a responsive site.
The app is something you install on your phone. It will be the RSC app and will only load the forum. So it will be easy to get to and use the site from your phone.
VBulletin is also responsive. Meaning that the site will change its layout, fonts, etc to fit on different size screens. We would just need to use a responsive theme. Here's a responsive demo.
Load it on your phone and desktop to see the difference.
So without the app you will still have easy access to the RSC with a good user experience. But it will be through your phone's or tablet's web browser. But the app might be more convenient.
Tapatalk will work with VBulletin, but it needs a plugin to be installed on the VBulletin site. We can't do this on their Cloud service because Tapatalk is a 3rd party plugin. So Tapatalk won't work for us.