"Illegality?" Wrong - that is precisely the issue. Otherwise, you are straining at gnats, and swallowing camels, when it comes to constitutional law...
Not at all. The question you posed is why anyone would believe a referendum would need to be held before secession could take place. The answer is obviously Virginia did, or else it would not have placed the requirement in the secession documents. The underlying legality of her actions, or the fact that Virginia ignored the requirements she herself laid out are irrelevant. It's the method she believed was necessary to accomplish her act that is the question. Obviously had Virginia only talked to you ahead of time you would have straightened them out.