Hey!
I would try not to panic just yet (although I appreciate that is easier said than done!). It is not uncommon for firms to send contracts or joining information quite close to the start date, particularly if they are still waiting for final checks to come through.
You have done the right thing by emailing them, and I'm not sure there's much more you can do right now. If you don't hear back by the end of the day tomorrow, I would give the graduate recruitment team a call (or send them another quick email if there is no phone number), just to double check with them. It is most likely just an administrative delay, rather than an indication that there's a problem.
Hopefully they'll get back to you soon, and best of luck for your vacation scheme next week!