Hi Petter,
Are you sure VSs have only 1 core allocated? This sounds strange to me as for HyperFlow to be enabled, you must have Dynamic Balancing running, and if that's the case, all VSs must have all CoreXL-FW cores allocated for them.
It may be that you're confusing cores with FW workers? It could definitely be that a certain VS has only 1 FW worker allocated for it, and when this 1 worker handles a heavy connection, HyperFlow will trigger, and use other cores to process this connection in parallel.
In general, HyperFlow will only trigger and use other cores if the overall utilization of the system allows it (lower than a certain threshold), which means that other VSs should not be affected by it.
Thanks,
Amit