There is no such word. The law might seem strange, but it's no so weird as to deny facts. If your signature is there, then it's there. Perhaps you didn't mean it, perhaps you were forced, but it's still your signature. Sometimes the important legal question is what did you mean when you wrote your signature.
Putting "V.C." before / after / on top of the signature doesn't mean that the signature "isn't there". The physical signature is still there. Vi coactus means "having been forced". You might be thinking of "vi aut metu", which means about the same thing. Please don't claim that an invalid signature is somehow magically "not there", when the signature is right there on the paper for all to see.