Hey!
Congratulations on the AC invite (+ upcoming vacation scheme)!!
I'd say it is completely fine to mention the upcoming vacation scheme if it comes up, but I wouldn't try to force it into conversation (although I appreciate it is an achievement!).
If you are asked something along the lines of "where else have you applied?", you should definitely answer honestly and disclose the vacation scheme. It is a positive sign that you have already secured a vacation scheme, as it shows that you are a competitive applicant, and trying to hide it might be a bit unnatural (particularly if it comes up directly).
That said, I can appreciate why you are concerned about the "scattergun" risk. I was in a similar situation where I had applied to lots of commercial/corporate firms, but had also previously completed a vacation scheme at a private client firm - it is mainly about how you frame those applications. I'd answer honestly about why you have applied to different types of firms, and you might discuss things such as:
- You have explored different firms to properly understand where your interests lie / keep an open mind
- Your interest in this firm is still clear and specific, and you can back up your motivations with evidence from past experiences
- There is a logical thread across the firms (e.g. culture, training style, exposure to clients, etc.)
You don't need to over explain the differences between firms, and I would try to follow a clear structure for your reasoning - for example, that the vacation scheme will help you develop your understanding, but that you are particularly drawn to Firm X too for Y reasons.
I found that it was all about balancing being open and honest about your experiences, but also keeping your answers anchored in why this firm specifically appeals to you. As long as you have clear and tailored motivations for the firm that you have an AC at, mentioning that you have a vacation scheme at a different kind of firm won't undermine your application.
Best of luck with your AC and your vacation scheme, I'm sure you'll do great!!