Instructions:Click the link, the voucher code field should be pre-filled with 'VP40' and the voucher password 'DISCOVERY'.
The deal is for both new and existing customers. If you have an account, sign in, if not then you need to make one and it will sign you up for the 'Discovery Club' (which you can cancel at any time) and you can choose the wine you want to redeem the voucher against.
The set cases are £89, so will cost you £49 after the voucher, however it does state you can choose wines from the website itself and redeem the voucher against those if you don't fancy the cases. (click on the 'mix your own' tab).
There is no minimum spend specified, but I think you have to buy a minimum of 12 bottles of wine. The cheapest wines you can buy are £5.99, so 12 of them would set you back £71.88, which is £31.88 after the £40 off (£2.66 each). You may not need to buy 12, but in the T&C's for other vouchers it specifies a purchase of 12 to qualify.
This is not something I would use personally (ew, wine!), but I was given the code and link after a VistaPrint purchase and thought there might be someone out there who could benefit from it.