I have just started to venture out using manual mode?

I have a Nikon D5100 and have been shooting in vivid. Is this a mistake? Or should I be shooting is standard?

Shoot in whatever you think looks best!

Most photographers would agree that it is best to shoot with the camera as neutral as possible, and would use standard mode. If you want vivid colours, add them in later when you edit the images on a computer.

But just because "most" people agree doesn't mean it is right or wrong. If you like vivid mode and it gives the results you want, then use it.

If you don't mind a little more work at the end of the shoot, I'd recommend shooting in RAW mode that way you don't have to worry about choosing a "style" in camera you might not like afterwards and you can apply that "style" afterwards in post processing.