Thanks again for the replies. I'm fairly sure I'll stick with VB3 for the time being as a result of the advice above.
Although the number of mobile visitors to our site is increasing, the main reason for wanting responsiveness is that Google seems to have downgraded our site's authority, probably because it's not mobile friendly. Having a mobile friendly version of my VB3 forum would awesome. Where do I look for this sort of functionality?
I must admit that our forum is very basic as I've always struggled getting my head around VB. We have VBSEO installed and one other 'plugin' that limits image sizes in posts. I also have my main site's navigation inserted in the header as php.
Is it a good idea for me to add a link here so you guys can see it?