P3R3 welcome to FT.
Unfortunately, it's a feature of the airline industry that market conditions are different in different places. Also unfortunately, this seems to be poorly understood in some markets, so we get posts which assumes that if a US based airline does something, that's how it will work throughout the world.
Like it or not, in Europe, a number of things are different. Our airlines put into 'taxes and fees' a number of charges as well, including things like airport charges, security supplements and fuel surcharges. This is pretty standard in the European airline market. It makes the airlines more profitable, even if it increases the price of our award tickets.
Also, I do have to wonder if you are comparing like with like. Part of the fees you are quoting for VS include Passenger Air Duty, payable on all departures from the UK - whether award or revenue. Anyone sat on a seat ex-UK (unless in transit) has to pay it. As such, to say DL only charges $13 when the minimum government tax is (IIRC, but I don't tend to actually look it up since its too scary

) £40 means either the British taxpayer is being ripped off by the US airlines or you didn't look at the same route on DL. I suppose the nice people at DL could be paying it on your behalf, but for them to do stuff like that does show why they aren't as financially secure as they could be.
Yes, DL will be cheaper in terms of taxes and fees. But you really should be comparing like with like - the UK is quite an expensive destination because of our passenger air duty charges. Welcome to the European airline market.
If you think the discount is small, then I would imagine you are looking at a Y redemption. It's usually not the best use of your miles, and yes, given how much of the ticket is the taxes, fees and surcharges, you may not find too much of a difference between redemption and economy price. That's why many of us over here will only use miles for PE/UC redemptions or for upgrades. Much bigger bang for your miles.
(BTW, we have a search function on FT which is quite good at looking to see if such questions have been asked before - this one seems to come up with monotonous regularity

).