Vauxhall is a major station. It is like a Waterloo East, may as well call there. Yes the Victoria line is there (you are in eg Euston or KXSP far quicker than going via Waterloo, and it has a HUGE benefit in not pushing people via Victoria itself)
It is also a destination of sorts - access to the South Bank, Millbank/Tate, tons of development (residential population is expanded massively) - plus nightlife, Oval... not least, a big bus station!
As for the stopping nature of the rest of it - 4tph to Reading and 4tph to Windsor would see adequate patterns, and frequency which could mitigate against slower trains. Both extras via Hounslow - Windsor could be a stopper.
4tph into Reading from the Staines direction could see some calls missed (certainly Longcross but maybe Winnersh Triangle, Martins Heron... could be fine with 2tph) - and if 2tph went via the loop, I'd go for Brentford only due to the development there.
I would flight the whole thing Japan style - local, rapid, limited express (semi & fast in our vernacular) - but the rounder via Wimbledon throws off being independent to a degree.
Also, could the main Shepperton service run via Richmond (maybe in lieu of the rounder) - to squeeze in another stopper which peels off as it gets caught up.
Twickenham's lines and platforms might have more potential to be used too.