You can't use any vehicle that could be mistaken for a Taxi.
That was why some Taxi drivers attempted to stop Vianos from being PH.
Because they looked like a Taxi.
Those saying it clearly forgot that the Vito Taxi had only been in London since 08 and the Viano/Vito and before that the original V Class had been getting used as a PH vehicle in London since the late 90's.
So basically TFL should not have allowed the Vito Taxi to be a Taxi in the first place. Especially since most have the RWS deactivated. They aren't much better on fuel than a well maintained TX4 anyway. That info comes from a mate that has used both. They may be a little better on a long run and are certainly faster but they are also fookin huge. I have done more than a few days in a Viano and they are fine away from Central London but well out of their comfort zone in parts of the West End or City.
A TX4 is actually quite enjoyable to thread round the streets of Central London, they are even fairly comfortable if you replace the shocks when they need it, which is every couple of years.
For starters, their indicators don't work and that's an MOT failure.
And they also suffer from a steering assembly design fault which could explain why they can't travel in a straight line.
Not to mention that it would also fail the emissions test, so in short NO.