The difference is that the Vulcans were ignoring Earth. If they had known that Humans were about to break the light barrier, then they would have kept a much closer eye on us.
It's not a good idea for the first time you meet a new species to be when you bump into them out in space. You know nothing about them, so you don't know whether approaching them with shields down will be viewed as a sign of friendship or as a sign of weakness. Doing your research on them before the encounter in space is a better idea - you know how to prepare for them.