Whilst xxx mentioned there may be no legal obligation to inform either firm about your TC or VS with the other depending on your agreement with both, I would ask you to reflect on whether it is morally right to do the vacation scheme whilst you have signed a TC. You taking up a place on the vacation scheme may hinder another candidate’s opportunity to shine and secure a TC with that firm. It may be that candidate’s “right place, right time” occasion after going through many application processes, and you completing the vacation scheme, whilst having a TC under your belt, instead of them might just rob them of an opportunity let alone waste that firm’s time with a candidate who won’t be training with them anyway.