Are you running heavy mysql? What is your vps IP I can take a peak or paste your ticket #
Sadly but true sites will run better on shared servers at times due to the fact they can burst 100% to use the whole server. However thats when other users service is compromised due to having server hoggers.
With the vps your limited so you don't affect other vps on the server. It is basically an isolated enviroment.
People seem to assume quick that vps is for hosting busy sites. That is not the case. It is more for more control over the server as a whole and to protect the smaller sites from the bigger sites who do hog the server.
I am not saying that is the case with your vps just kind of talking out loud
