Don’t be worried about the value you can add during a vac scheme, as ultimately your end goal is a TC and a (potential) career there. Future thinking is a requirement.
You are correct when you say about highlighting your skills and desire to pursue a particular area of law. For me, my personal transferable skills from my STEM degree, my in-depth knowledge of my sector of interest (healthcare, biotech etc) and my commercial interest (and the reasons behind it) are key talking points when saying ‘why me’.
What skills do you have which will make you an asset to the firm? Don’t be vague, make it personal and make it relevant to the firm you are applying to. In doing this, your application should be unique and interesting to read, which is crucial as it is a human who is reviewing the applications after all!
I have realised that I haven’t really said how you can add value, well in my opinion (for what it’s worth) take your skills and compare them to legal practice sectors (maybe your favourite practice area?) and see how they overlap. For example, I enjoy analysing data, weighing up different opinions on a concept and arriving at informed conclusions, which to me relates to dispute resolution.
Also, are you knowledgable about say aviation, gaming, life sciences or the energy sector? If your firm is big on those sectors and you know a lot about one or two of them then that will be an advantage when dealing with clients in said sector, right?
Anyways, hope that helped despite my answer being rather unstructured and likely convoluted.
