As stated, I had heard Vans Direct offer good deals but answering your question, Vanarama are basically a broker and use larger leasing organisation to finance their deals such as Lex, Arval, LeasePlan amongst others and they then add on a commission. This does not mean that the actual finance organisation is cheaper approaching them directly - they may or may not be. Much depends on their own internal policies and contracts.
As with many 'headline' promotions and in this case for leasing the devil is in the detail. Do not just look at the headline rate.
Get comparison quotations on a like for like basis (same vehicle/specification, term, mileage, payment profile etc with any ancillary solutions of your choosing such as racking and livery included). Compare.
Monthly rentals are just one of the considerations - happy to list others if you are not familiar with leasing.