vnc, while easy to use, is heinously full of holes in its OSX system-level implementation and is easy to break into. if you're serious at all about data security VNC is a non-starter on OSX. it does NOT employ the same level of security that standard Apple Remote Desktop protocol employs.
I have little snitch running on my mac. for giggles, i implemented VNC connections for a short amount of time. the amount of pings you will get on port 5900 for access is astounding. there are people out there just bulk-scanning ranges for a response on 5900 at which point they just start trying to brute force because of the weak auth mechanism. i would never, ever, ever allow Apple's stock VNC implementation to have open access to the internet. never if, always when, the point at which your machine will get compromised.