I'm a vbulletin guy myself (aka Princeton).
We humans hate CHANGE.
We are talking about a MAJOR change here. vBulletin and Discourse are nothing alike.
You will have a huge BACKLASH in your hands.
The reason I say this is because...
It's evident that you don't make big changes to the vbulletin UI. So, any big change to how people use the site will not be welcomed.
Your audience may not be ready for such a major change. What have you done to prepare them for such a big change?
Your site and Sitepoint are not equals...
Sitepoint is known to constantly make changes to it's forums (more than the average) - especially the forum "homepage". They definitely made a lot of changes to the site as a whole in the past few years. They are constantly evolving and members now expect change as the norm.
The Sitepoint audience are more acceptable to change than the average group.
Change is inevitable. We need to evolve to grow. I'm glad you know this.
It's great that you ask your members for feedback but it's important to know that sometimes what they say is not what they do. Review your analytics - take a close look at what your audience is doing.
What's your best channel? Organic traffic? Social? Direct? Referral?
Focus on your best traffic channel. I'm not saying to ignore the rest. If Organic Traffic is your best channel then make sure that you don't do anything to negatively affect this.
To increase activity/traffic, read the messages that members leave on your site (not about site feedback but in general) you may be surprised at what you find. Ideas are often found where you least expect them.
What are they looking for? What do they want/need? Satisfy their need(s).
To prepare your audience for a Discourse upgrade I recommend you start making incremental changes now. Get it as close to what you believe your Discourse installation will look like. Do this in a 6-12 month period. Before you know it, your members won't even notice that you upgraded to a new system. Your members will be talking about the awesome new features -- Not about the "ugly changes".
If site is slow ... You may want to look at a master/slave setup if you don't already have one. (Personally, I found the speed ok.)