VPS is presumably much of the reason.
Regardless of the marketing claims, a VPS is going to be far more limiting than a dedicated server. Nothing against VPS per se, but load limitations / uneven loads is going to be issue when sharing a traditional VPS server.
Amazon's EC2, while similar in some aspects to a VPS, is structured differently and hence may be a better solution, in particular, for larger sites that want to be able to smoothly scale up and down in responses to loads. With that said, that's likely overkill for what you're seeking, but anyways, for more details see
http://aws.amazon.com/ec2/
First thing to do is find out what the process, CPU load, memory limits, etc are for your VPS service (details likely buried in the TOS / welcome email) - you may be able to make simple changes to reduce the load.
Also, many VPS services, especially for new customers, will assist in finding out what is causing the higher load and, especially if it's due to other users / server overloading, will offer to move the account to a less loaded and/or more powerful VPS server.
And finally, if anticipating much growth, price out dedicated servers - if you're handy with server administration, that will greatly reduce the cost. Otherwise, go with managed - costs a lot more, but far less problems, and better support.
Hope this helps.
Ron