I've read this in several grow books at barnes and nobles, so it's a reliable source:

As soon as the first true set of leaves emerge, the plant is technically in the vegetative state, as it's absorbing light, etc...

The plant is no longer a seedling, once it's at it's 5th node. Then it's considered a vegetating plant, but even when it's a seedling, it's still vegetating, so as soon as its first set of true leaves emerge, it's in vegetative state, and as soon as it's at it's 5th node, it's not a seedling anymore.

Kind of like, how babies are still alive once they are born, but you aren't considered an adult until you are 18. Well, no, that makes no sense after typing it.