Hello!
Congratulations on the Slaughter and May HK VS - that's a great achievement!!
In terms of whether to mention it in applications for VS in London, I think it depends on how you frame it. The VS itself is impressive and demonstrates capability, but firms will want to understand your trajectory and reasoning, especially if the experience doesn't directly align with the practice focus or region you are applying for.
If you decide to mention it, I personally don't think it would necessarily harm your application - it can show you are a strong candidate who has been competitive enough to secure a VS at a leading firm. The key is to connect the experience to what you're now pursuing. For example, there may be similarities in other aspects of the firms, such as international work, culture, pro bono, exposure to high-end work, etc. For example, I listed my upcoming VS at a private client focused firm during my Reed Smith DTC application - whilst they didn't have similarities between practice areas, I was able to draw similarities in other areas which helped show my reasons for applying.
However, if you feel you can't draw a meaningful link and it risks creating confusion around why you're now targeting US firms in London, then leaving it out is also perfectly reasonably. You're not obliged to list it if you haven't completed it yet - it's clear that you are able to be successful in applications, so you don't necessarily need to list it in order to be viewed as a desirable candidate!
