The one that matters most is OvmsPoller::m_poll_state - as that is the one used in processing. It is possible to set this separately per bus.
The one at OvmsPollers level is there if you choose to keep one 'poll-state' and so set the values on all busses together... which is normally the case. It's mostly there so multiple calls to setting poll-state without actually changing the poll-state will not actually have to queue calls.
The one at OvmsVehicle is there because that used to be where it sat. It's still there for reading convenience and for non-blocking access to the value.. but it really should always just be a reference copy of the m_poll_state on OvmsPollers.