WIJG: "Illegality?" Wrong - that is precisely the issue.
N-S: 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.
LOL! Are you suggesting that the federal government used military force to suppress the secession of the Southern States (resulting in the loss of 600,000+ American lives), because those States in some way violated State laws?
ROTFLMAO!
Not at all. The South resorted to military force when they bombarded Sumter and initiated the armed rebellion which followed. It was that rebellion that the federal government suppressed.