Sorry to be boring.
95% of the wheels aimed at the car market do not have an adequate strength rating for the Vito. This includes wheels offered by retailers as being a fitment for the Vito, they make this assessment purely on physical dimensions. I spent hours looking into this as part of an insurance assessment and spoke to many experts and manufacturers.
No-one ever wants to hear about this, but it's true. Wheels should match the axle rating of the vehicle. The Vito is 1550Kg, or 775Kg per wheel.
Everything you see in Halfords tops out at 400 - 600Kg per wheel.
That's a difference of 375Kg per corner, or put another way, 4 adults extra per corner. Weaker wheels won't bend or show their weaknesses so they could be fine for thousands of miles. Aluminium Alloy doesn't work that way - what happens is that stress fractures appear over time, and one day you hit a pot hole and the wheel ribs could fracture and rip the rim off the hub.
My point is - don't trust the retailer or the manufacturer - even their catalogues. Commonly heard question from wheel retailers and even manufacturers "Will you be loading the van fully? If not these wheels will be OK".
Rondell and even OZ asked this question. This however is complete bollocks as insurance companies will never buy this.
If the wheel isn't stamped with its weight rating it ain't rated. EVEN if it says TUV approved.
Same applies to tyres.
No matter what your tyre fitter says, speed rating has little to do with weight capability. Simply put, the speed rating denotes the tyre's ability to retain its integrity when faced with centrifugal forces acting outward as the tyre spins. Choosing a tyre capable of travel at 170mph for a vehicle with a top speed of 120mph does not automatically mean it is extra safe for a Vito. That is only one part of the story.
The load index is the one to be interested in. This denotes the tyre's capability to withstand downward force.
Many low profile tyres such as the Toyos and Fuldas have a load rating of 96Y.
This means they are suitable for 186mph. Wow! But, the "96" part means they have a load index of just 710Kg, 90Kg short of the Vito's needs. That's down by a whole extra full grown man on each tyre! 4 adult passengers is a hell of a lot to be out by.
The stock tyre has a load index of 101, or 825Kg. We have chosen 100 rated tyres for our vehicle, or 800Kg. Which still gives a greater than 10% safety margin over the 775kg plate weight.
Tyre retailers will often describe a tyre's strength in terms of the number of plies in the sidewall. But, it's important to note that no matter how strong the tyre actually is, unless it is stamped with the correct weight rating, it is illegal in many countries, and will invalidate insurance. Number of plies is not a reliable indication. If the tyre is stamped, this is a statement that comes direct from the manufacturer - who is afraid of being sued, and should be considered as far more reliable safety information than the opinion of a retailer.
Tyre diameter is the final thing. When changing alloy diameters, it's matching the outside diameter of the tyre that's important, and manufacturers don't quote these figure in catalogues or on the sidewalls. Most people know that the speedo will be affected if the result is wrong but with modern vehicles there's a more serious concern - the ABS computer will also be affected as it is programmed with the OD of the stock tyres.
Sorry to be the bringer of bad news. Hope it's useful.
These guys have rated alloys:
Mercedes Vito Van 03 Wheels Tyres