I think it depends on the contract you sign with your law firm but also if the WVS would allow you to have accepted a TC as some law firms do not take on students who have accepted one.but would it really be too long of a delay? i'm mainly looking to apply for winter vac schemes, and the gap between the vs i have lined up and the ones i wish to apply for are at most 6 months. i also would still be a student by then as i graduate in 2027, so if i was to train at the firm i have a vs at i wouldn't start until a year after i graduate. and i was under the impression that some winter vac schemes only accept 3rd years/graduates so they can begin training earlier than people who do summer vac schemes.
It also depends on whether you will receive financial support directly after accepting your TC (sponsoring PGDL, SQE) as you would need to repay what you would have received.