Just building on what
@Jessica Booker said in response. It’s great that you’re considering both the open day and the vac scheme! Your concern about potentially waiting a long time to hear back about the open day and the impact that might have on your vac scheme application also makes sense. Like Jess, though, I'd suggest waiting until you've made the open day application, especially if you feel that attending the open day will give you additional insights and help you strengthen your vacation scheme application.
Alternatively, I don’t think attending the open day is strictly necessary to write a compelling vac scheme application. If you've written strong, well-researched answers, there's no harm in applying for the vac scheme now. For context, when I was applying to
Cooley, I was unsuccessful in securing an open day, but made it through to the vac scheme during the same application cycle. Whatever you decide, best of luck with both applications! Feel free to reach out if you have any more questions