You wouldn't need native iOs and Android apps - ya know, the ones you download and they stay on your device.
The website could be re-written in HTML5 using media queries to dictate the screen layout. Basically HTML5 is capable of detecting the screen size of the device you're on, and adjusting the screen layout accordingly. Go to www.jimmyjohns.com in a web browser, and then start dragging the web browser window smaller and smaller. Watch what happens. Eventually when you hit a certain size, the media query tells the website "oh this is a mobile device", and it switches layouts (more vertical and less horizontal, more menus and less hanging out on the screen, etc). You can see that on jimmyjohns site; once your browser window is about half size, it thinks you're on a mobile device and switches to a completely different type of layout that is optimized for mobile.
However it's still just one page and one code base, so it's just as easy to maintain.
The other cool thing these days is that the browsers are coming with their own built-in databases. You could set the site up to work off this local storage (e.g. IndexedDB), and then sync IndexedDB to the server when the device is connected. This would allow for browsing and even posting to the rx with no internet connection. Then when the device got back in range, your queued posts sitting in local storage would get automatically sucked up to the server.
The other thing I would look at if re-doing the website would be to implement signalR technology to allow the server to push to the client. For example, if you were sitting on the RX homepage and someone started a thread, the new thread would pop up on the screen without a refresh. Same thing if you were within a thread and there was a new post. Would make the site more "live".
IMHO it's time for the rx to start looking at some of these technologies. Just this year I led a team of 2 programmers that re-wrote our biggest company application to an HTML5 web app with local storage, and it's amazing, and it only took 2 guys 3 months. Works across all devices, laid out nicely for mobile, and uses a single code base. We even implemented signalR so that if the analysts start a new job in the system from the office, the job shows up on the tablets of the techs in the field with no refreshing.
I only tell you this stuff to convey that this would not be ridiculously expensive or even time-consuming, but I'm not sure how nicely vbulletin plays with HTML5...I'm assuming they have made it nicely compatible.
I post a fair amount from my phone, but I'm sure like everyone else I get frustrated because the screen doesn't resize properly, so I'm scrolling side to side a lot, zooming in on buttons to click them, etc.
