If making direct tc apps and mentioning that you have a vac scheme in the summer coming up, do you think it would be helpful to include a brief one liner that your still mostly interested in the firm your making a direct tc app to, for a training contract?
This is where I think the adage 'Show, don't tell' comes in. Simply telling the firm that you are mostly interested in them is meaningless. As
@Jessica Booker said, the firm knows that you wouldn't be applying if you weren't interested in them, but they also know that you'll be happy wherever you get an offer!
The only problem is if your written application itself is not compelling enough to convince them that you are mostly interested in them. If you explain all of the reasons why you like them well enough then you should have them believing that they are your dream firm without question!
If it comes to an interview stage then you can start preparing for a possible question about what happens if you get both offers, but at the application stage it may even be a red flag to imply that the firm should be asking questions about how interested you are really in working for them.
I hope that is useful!