I used to think that a VPS would get it done for me too. I even upgraded to a more powerful VPS solution. The bottom line with a VPS is that you have many accounts on the same server competing for the RAM and the CPU's. My forum is approximately the same size as yours and I simply had to go to a dedicated server about a year ago and I'm very glad that I did. Of course, I run a number of additional sites but the real resource hog is my vBulletin site. I know it seems like a lot of money to spend but your user experience will increase dramatically. Even a low end dedicated server runs circles around a VPS account. All those specs about "guaranteed" RAM and the like are bunk. What really eats you up is CPU.
For best bang for the buck, I have a dedicated server with
JaguarPC and I have not been disappointed.