if a VI question asks to discuss a recent story I've been following / opportunities and challenges currently affecting the legal industry, is it good practice to always link it back to the firm and how it can affect them/their clients?
I think it's definitely good practice to link it back to the firm. You're trying to show you would like to work there in particular, after all. If it's a question that specifies the "legal industry" then why wouldn't you link it back to the firm, a business that operates in that industry?
Even if the firm you're applying to isn't directly affected by some change, there will probably be indirect effects you could mention - how do these 'opportunities and challenges' affect competitors? Are other firms gaining a competitive edge? The reverse?
I suppose it's easier to answer with a brief example. So, just from the top of my head - the SQE could be seen as an 'opportunity' that will affect the legal industry. How solicitors qualify affects all firms. In theory, the new qualification should lower training costs. How will this, in theory, affect firm profits (long term vs short term)?
But even with a broad story such as this, you can make explicit links to the firm you're applying to. How much does the firm invest in training? If the firm you're applying to has a large trainee intake, it will presumably save more than one with a far smaller intake - specific opportunities for reinvestment, or negligible impact? Will this benefit competitors more than your firm? (There's plenty more to say about the SQE, of course, but too much for this reply...!)
Just a random example (and without much depth!) but it hopefully shows that broad changes to the legal industry will have some effect on the firm you're applying to. Tailored answers are generally better than generic ones, I believe!