A First is 70% or higher, so you're sitting on a high 2:1. That's perfectly fine. You would need someone who is more familiar with the inner workings of vacation schemes to confirm (I'm a barrister so have very limited practical knowledge of them), but I can't imagine that a low mark in Contract Law in your first year would scupper your chances of obtaining relevant experience. It's also worth noting that life as a solicitor does not begin or end with Magic Circle firms, and in fact for many they are a positively bad choice given the working environment and expectations. Even if you're fully informed and believe that the Magic Circle is right for you and cannot obtain the precise experience that you want, you still have plenty of time to get some experience now, bring your mark up to over 70%, and gain better experience in future.
Law degrees are difficult, and the transition between A-Levels and degree level study is one that takes time, even for the most able students. It's a transition that took me a good couple of years in practice. Your first year marks so far are very good. Whilst there is room for improvement, you should absolutely be proud of those and not feel that you've failed or in some way not met expectations. You've made an excellent start. Keep it going.