it will lighten all the clips you put on that track, because this is what you told Vegas to do - the effects you add to the box on the left will apply to the whole track. make another video track and drag there everything you don't want to put on lighten - this is what everybody does.
I know there is a way to set only one clip on lighten/darken/etc. without changing the properties of the entire track, but I'm too lazy to figure it out. you should ask someone else about it.