Could always argue semantics and say a PV isn't an e-cig I suppose but that would depend on whether one didn't mind being turfed off the bus and end up having to walk.
They've got the right to ask people not to vape on their own property, but you'll note, it says nothing about legal consequences/fines because they have basis to impose any.
2 ways to combat it if unhappy :
Militant approach: Ignore the sign, vape on and face the consequences of being turfed off the bus.
Educational approach: Send Arriva literature regarding their perceived risks to other passengers from vapour and hope they might learn something.
Other than that the only alternatives I can come up with are drive your own car/motorbike, get a taxi, cycle or walk to where you want to get to.
I know it hardly seems fair for them to ban vaping without a valid reason, but it's their property, their rules.