I think you'll find Tesco vouchers are only usable on IoW, Irish Sea and a North Sea ferry, only Eurotunnel being available for the short Channel crossing.
(Edit) I'm assuming you're taking a caravan as you mentioned the Caravan Club, but if not, have a look at the DFDS Dover-Dunkirk offer I mentioned here.
We booked with DFDS this year as they were over £100 cheaper than P&O. Eurotunnel were the most expensive, but you can use Tesco vouchers. I just couldn't justify the extra expense of the tunnel. In actual fact the saving by using DFDS gave us a free day out at Legoland via Tesco vouchers.
We use tesco vouchers for the tunnel, they cannot be used on channel ferry crossings. Cheapest ferry to go for is Norfolkline Dover to Dunkirk, normally around October there are discount codes about for the following year. You can normally find these on the camping abroad section.