I will be starting my vacation scheme after the deadline for submitting training contract applications has passed. Is it ok to still mention this vacation scheme in the work experience section of the

Definitely. When making training contract applications, you should include both past and future vacation placements. They are an important way of proving your interest in the legal field and your desire to want to learn more about becoming a solicitor. For schemes you have completed, you should detail on application forms what the vacation placement entailed, which practice groups you were able to experience and what the placement taught you in terms of a legal career.