One of the problems I've always had with any VPS was that you're paying so much for bandwidth and disk space; when you don't need it, simply because you want more ram/cpu power for your database driven website. It was always very expensive, and not as fast as you wanted it. I never ever reached even close to 10% of disk space and bandwidth usage, but I always peaked server load.
Well my host just recently started offering SSD VPS options. They only have about 60% of the disk, which is fine, since I don't even need 1/3rd of that; but they have a massive increase in performance! Not only that, but the costs are 40% less than what I was paying for my HDD VPS! I highly recommend you guys try out SSDs for your XenForo forum.