Hey!
I have done both, and I would say that DTC AC's typically do have a higher benchmark as the firm isn't able to assess you over an extended period of time like a VS. This can sometimes mean that the standard of performance is higher, and I have found that they usually include additional tasks as well (to assess you across more competencies).
With a vacation scheme AC, as you mentioned, the firm knows that they'll have a couple of weeks to get to know you, so there is sometimes a bit more room for potential. They can see how you work in practice, how you take feedback, and how you develop over time. With a DTC AC, they don't have that follow-up period, so they are usually looking for things to come across more clearly on the day.
In practical terms, I found that this means your answers need to be more polished and well-formed. Your motivations should feel really clear and backed up with evidence (e.g. why commercial law, why this firm, why you), your competency examples should be a bit more developed and structured, and your commercial awareness should come across in a more practical and applied way - instead of just showing that you know what is going on, you need to be able to explain why it matters for clients or the firm. They aren't expecting lots of legal experience, but they are expecting you to show how your experiences have made you suited to the role of a trainee solicitor at this specific firm.
On the VS vs DTC point, I do think it is fair to say that firms can take slightly more of a "wait and see" approach with vac schemes, whereas with DTC they need to be a bit more confident upfront. However, I don't think that means that the bar is wildly different, and plenty of people who don't convert a VS AC go on to succeed at a DTC AC (myself included - and I don't think of myself as particularly confident in those sorts of assessments!).
If you have had a VS AC that didn't go your way, I always found that these were really valuable in shaping how I prepared for the next one. Things such as being more structured, more concise, and a bit more deliberate in how you link your answers back to the firm and its work are all little things that can make a huge difference. There is absolutely no reason why you would fail to convert a DTC AC just because you might not have converted a VS AC in the past - different firms have different benchmarks, and it might just be that you weren't the right fit at the time (+ you might be for this firm!).
I hope that offers an alternative perspective, but happy to chat about it further if you have any other questions!