VAA's contain two sections of data. The header section metadata about the current VAA and are mutated as the signatures are aggregated. The lower section second contains data that is deterministically derived from the message and therefore immutable. This second section is the data that a guardian must sign. As this data is deterministically derived, each guardian is able to produce signatures without communicating with other guardians. Over time the signatures will accumulate until a consensus threshold is reached and the resulting VAA can be submitted on chain.