Not sure when the DVLA introduced these, but I think they are fairly recent. I just went through the government website to set mine up - all reasonably smooth, wants the normal government data for verification (passport number, driving license number etc), and I soon got the account up and running. I then tried to add my car to the account - and it wouldn't accept it. It turns out that the DVLA Driver and Vehicles account uses full names (to line up with government ID), whereas my V5C is in my first and surname only - so the account glitches and won't accept it.
I went though the customer services to find out more about this, and the solution is for me to send my V5C off to the DVLA with Section 3 completed with my full name, and a covering letter explaining that this is indeed me, and I need the V5C updating because I need the name to match for the customer account. They will then send me a new V5C (after about 4 weeks to process, apparently), at which point I can log in and add my vehicle.
So - be aware, it seems that full names are needed on the V5C to be able to use the new online accounts.
I went though the customer services to find out more about this, and the solution is for me to send my V5C off to the DVLA with Section 3 completed with my full name, and a covering letter explaining that this is indeed me, and I need the V5C updating because I need the name to match for the customer account. They will then send me a new V5C (after about 4 weeks to process, apparently), at which point I can log in and add my vehicle.
So - be aware, it seems that full names are needed on the V5C to be able to use the new online accounts.