We have only one van - not something we need - we have it as a marketing thing. Because a lot of our labour is labour only subcontract they have unmarked vans, so we have one of our own to make appearances as required, and for site managers to use to set up /clear sites.
We are of the opinion that on first and last day on site a nicely signwritten new van matching up with the site signs looks professional.
If we as main principal contractors see a smart van with the right info on it we always make a note of the name for future reference. You just need to be careful what you put on there.
Mobile numbers only are a turn off, as are the same number for phone/fax. Separate lines or just a phone land line are preferential for creating the right impression.
A good web address is the most vital info on a van these days. It's easier to remember than a phone number and 99% of potential customers will track you down off it if they need you.
As a customer then I look for age & condition of van [if your van is well looked after it implies you are generally tidy and concientious], company name, credentials [trade associations if applicable] web address, phone number.
Turn offs are mobile numbers [better to have a land line with a divert] old vans, very plain or very naff sign writing or pictures.