Vaults can never be trusted to be honest. We have to expect they will be dishonest, in fact. With this in mind you either don’t trust them or you spot test them to confirm they are truthful.
With that in mind, we have to assume that available vault storage cannot be trusted. Any algorithm foe changing the reward amount must therefore be based on areas we don’t need to trust. Specifically, that they a vault stored the data and was able to confirm that was delivering it at a random later date.
If you blitz the network with tiny nodes and little storage in each, building up rank will take a long time and farming rewards will be limited. If you have one big node which gains rank bit then stops delivering the data it said it had, rank will be lost, as will farming rewards.
The farming award just needs to balance these. If a vault always stores data requested and always returns It on request, you can work that into the farming rate algorithm - lots of highly ranked nodes is likely to mean lots of storage availability in the future too, for example.
The important point is that the farming award can be adjusted to suit conditions. There are many ways you can measure these conditions and I am certain many will be explored and tested to destruction before any live safe coin is released.