The way I see it, when discussing your interest in the firm in a cover letter, you do not need to explicitly link features of the firm you find attractive to opportunities you would have in the Vacation Scheme itself. Instead, what I ended up doing in my cover letters (and what I know quite a number of other successful candidates have done in theirs) was to simply express an interest in those features of the firm in and of themselves (eg. "I am applying to X firm because of its strengths in M&A and PE, as these are the practices I am most interested in."). Thus, we would talk from the perspective of a prospective trainee rather than that of a prospective vacation scheme attended.
To the extent any recruiter would expect the latter, I still think this would likely not be a major issue, as the link between there being a feature of the firm which makes it an attractive place for your TC and you wanting to explore it in the VS is a clear and obvious one. As such, recruiters would not generally expect you to waste word count by repeating the same line over and over again even when the meaning of your points would be clear without it.