Does seem a little shady that the dealer hasn’t provided firm figures yet for you to scrutinise, yet expected a deposit straight away when presumably they have no intention of ordering the car at point of deposit taken.
Go through those figs carefully. I’m always wary of anyone selling something on a “how much can you afford per month?” basis rather than telling you physical costs of the product, interest etc. Look at the cash price and then see what VW finance does to that. Only by looking at the cash price of your order vs RRP can you decide whether you’re getting a good deal (via discounts or a favourable p/x value). If you “can afford” more than the car would cost with a decent p/x at full RRP, they could knock your p/x value down to fit the ceiling of what you’re prepared to pay on a monthly basis, or add in non-required optional extras.
Most dealers will give you a GAP valuation for your car now, some give you a price based on what it’s worth now, or in the near future. On a 3 year old car that’s likely to be p/xing in the £10 – 12k range, depending on which engine your match has, and whether it is 3 or 5 door (assumed good condition due to the low mileage), you could find your p/x price varying by as much as £1500 between dealerships, depending on what criteria they use to value it. Expect a very generous p/x value if you are getting little or no discount. Have a look at their used stock, find one at your spec or closest to it and see what the price is. A “fair” p/x value is usually at around 80% of the sticker price of what it will be up for, so if they have one up for £15k, I’d expect to be offered £12k in p/x.
You can get a free GAP valuation on Vauxhall’s website (you usually have to pay a few quid for one). They determine a GAP p/x price for you, on the assumption that you’re looking to buy a Vauxhall – so you may get a few emails from eager dealers. Your car is probably depreciating at the rate of about £1000 every 6 months right now, so if you do get a part-ex offer for £1000 less than GAP, based on a 5/6 month delivery, it’ll be to give a valuation that’s relevant to the date of new car collection. If they can get you a GTD within 3 months, I’d expect them to only understate your p/x worth by £500 etc. If they only give you what your car will be worth at pick-up point, they’re giving you nowt unless they give you a list price discount.
Overgenerous p/x value or significant discount on the car (maybe both!) is what you should be aiming for.