It depends on how much ram your vps server has. The more ram the more traffic the vps server can handle.
Though I will say that a vps server with 1 GB of ram will handle or should handle 5,000 daily visitors with ease.
It should handle up to 10,000 daily visitors with ease as well.
When you get to 10,000 daily visitors I'd look at getting a dedicated server as you should be able to monetize your website with Adsense or something similar and earn money to pay for the dedicated server and more.