It sounds like your cable is shorted / bad.
I have the Canon version. I can tell you the camera side is a 2.5mm stereo headphone jack -- but not being sure which Vello remote you have (or being able to look at it) I don't know what the other side is. When I look at their wireless "airwave" remote it looks like it has the same jack on both sides.
In any case, it's a _very_ simply mechanism. There are three wires inside the cable and the "stereo headphone" style jack has 3 conductors on the post.
If the shutter button is pressed half-way then the middle and end farthest from the tip create a connection. If the button is pressed all the way, then all three create a connection (tip, middle, and base of the pin.)
If the camera stars triggering the shutter just becuase you plugged it in, then that means something is conducting across all pins -- and given that this doesn't happen unless the cable is connected, it almost certainly points to a short in the cable.
Replace the cable. You may have to contact Vello or a store that sells them. They do actually sell *just* the cable (without making you buy the whole remote to get one.) This is because you can buy extra cables for cameras that require different types of connectors.
Tim Campbell
5D III, 5D IV, 60Da
