Posted about this before. Unfortunately no one size fits all approach to this - different firms will operate different policies/approaches about the timeframes you could be given. Some will give you a lot less than a month to consider the offer.
However, you approach should be as follows:
1) Ask for an extension of the offer - particularly given current circumstances, I expect firms may be more flexible this year
2) If they say no, make a decision to either:
1) accept and withdraw from your vac schemeLaw firms will hate me saying this, but there is very little stopping you from accepting a job and then reneging at a later stage. Just don't do it if you have received financial support from the firm. At worst the risk is that you won't be able to apply to them again in the near future (e.g. upon qualification) but even then that risk is actually pretty low.
2) accept and continue with your vac scheme (check your T&Cs of both offers though to ensure this is ok)
3) decline the offer and continue with your vac scheme
Reneging isn't great - but plenty of graduates do it each year. Plenty of people in other forms of jobs do it. Just one of the negative things of recruitment.
This is super helpful! thank you so much!