Hello!
Congratulations on the vacation scheme, that is fantastic news!!

I can totally understand what you mean, as I definitely felt like I hadn't prepared enough going into my schemes - mainly as I didn't know where to start!
However, what you are doing already sounds exactly right, so try not to worry too much! There aren't actually that many things you can do in preparing for the scheme, so I would focus on:
- Having a solid understanding of the firm (e.g. key practice areas, recent deals, culture, unique features, and its competitors)
- A basic grasp of the departments you'll be sitting in (what they do, typical clients, current trends, etc.)
- Keeping up with a few commercial stories, both for any interviews and so you can chat comfortably if it comes up
More importantly, a lot of success on a vacation scheme is about how you show up during the scheme, rather than how much you know. Therefore, during the scheme, I'd make sure to:
- Be proactive - ask lots of questions, see if you can assist others with work, and show enthusiasm
- Ask thoughtful questions about the work, but try to think things through first (so you can show you have tried to find the answer yourself)
- Be someone people enjoy working with - friendly, engaged, reliable, etc.
- Stay organised and meet deadlines, even for small tasks
- Communication is really important on a VS - if you have a session, make sure to let your supervisor know so they can assign an appropriate amount of work
I also would avoid putting too much pressure on yourself to be perfect. Firms aren't expecting you to know everything (even if it feels that way!) - they are looking for potential, attitude, and whether they'd want to work with you. Ultimately, they want to see that you can take on criticism and are willing to learn, as that is the main goal of a training contract!
You are definitely not stabbing in the dark, though I can definitely relate to that feeling pre-VS - you are already doing the right things! I'd just ensure that you have a good understanding of the firm and current commercial events, and can discuss your motivations towards law + the firm clearly and concisely. The rest you can pick up very quickly once you are on the scheme - wishing you the best of luck!