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Remote administration tool
There's very little chance that even the best programmer could modify VNC into a trojan, without rewriting so much that its more taxing that writing it from scratch.
If you're looking for legitimate RAT, just use BSD and SSH. If you're admittedly a skiddie looking for a trojan, try evileyesoftware.com's Optix. The whole point of an AV is to catch that stuff, so don't cry when a trojan is detected.