Hi!
Congratulations on the vacation scheme, that is amazing news!!
I can totally understand how you feel - I also went to a non-Russell Group university, and was often one of the only non-RG students on the scheme too! Imposter syndrome is incredibly common on vacation schemes, and everyone around you will likely be feeling the same way (even if they are Oxbridge!).
The most important thing to remember is that the firm chose you. Securing a vacation scheme is highly competitive, particularly at US firms, and they would not offer you a place on the scheme if they didn't think you had genuine potential to convert. You are on the scheme as you have earned your place on merit, not by luck!
It can also help to reframe the comparison that you are making. A university name is only really one line on a CV - what firms assess during a vacation scheme is how you think, communicate, approach work, and whether you are someone they would trust at 2 am on a deal. Those qualities are not necessarily determined by where you studied! I actually found that the fact my university was 'less academic' helped me in some ways, as it meant I had completed a placement year and been involved in other extra-curriculars that taught me valuable skills.
Additionally, try to focus on what you can control! Make sure to prepare properly for tasks, ask thoughtful questions, be proactive (whilst avoiding being overbearing), and actively seek feedback so that you can improve across the scheme. Overall, what matters most is showing enthusiasm, reliability, and a positive attitude. Your academics have clearly been enough to get you on the scheme, so the firm thinks you are more than capable of doing the role! You now have to show that you are a good fit for the firm and have the right attitude and approach to a training contract (i.e. that its a learning opportunity!).
It is also helpful to view the other vacation schemers as future colleagues, rather than competition. The graduate recruitment team will notice those who contribute positively to the group dynamic, and I always think it's important to remember that everyone is probably feeling the same anxiety as you (despite the fact they may be a RG or Oxbridge grad, or have lots of legal experience). You don't need to try to outshine everyone in order to convert - its more important to be consistent and willing to learn!
Imposter syndrome tends to surface when you are outside of your comfort zone or trying something new. This discomfort doesn't mean that you are not good enough at all - instead of thinking "I shouldn't be here" or "I'm only here due to luck", remind yourself that you were selected for a reason and you have all that it takes to succeed.
Wishing you the best of luck for the vacation scheme - I'm sure you'll do great, and I hope you enjoy it too!