There is indeed a circuitous route easement meant to prevent this, 030230:
I'm not Sure why it wouldn't be applying this though...
Why is that easement deemed necessary? Fares from outside London to Vauxhall (other than when a passenger uses a Zone 2-6 period travelcard) require a ticket to be purchased to London Terminals so there is no loss of revenue for a passenger going via Waterloo.
Indeed, given you can't buy an explicit ticket to Vauxhall from Guildford, does the easement apply when using a London Terminals ticket? In this instance, the traveller from Guildford to Vauxhall will actually be buying a ticket to London Terminals
https://www.brfares.com/!locations?dest=VXH
(Link shows that only 139 origins are defined for tickets that have Vauxhall as a destination)
Origins available to
VAUXHALL
Shown below are all origin locations that have fares defined to VAUXHALL.
VAUXHALL is additionally a member of the LONDON TERMINALS fares group.
Is the easement actually that you can't circulate via Waterloo with a ticket issued to Vauxhall (priced on Vauxhall being in Zone 2), but can circulate with one issued to London Terminals (priced on Vauxhall being in Zone 1)?