Accessing a server via VNC requires that the server already have an OS on it. Which pretty much means it's impossible to install an OS via VNC. I'm not entirely certain how you "installed CentOS via VNC" in the past, but I'd certainly love to see a link showing how you did it. (If you could see my face, you'd see "incredulity"
)
That being said: There are some remote access systems available from various manufacturers that allow Remote KVM access. An example is a DRAC card (Dell's remote access method). These come with challenges and some work better than others ... but if you can get someone to locally plop in the .iso image CD, it usually works out fairly well (if your server has a DRAC or other Remote Access method, of course).