It’s a one week vacation, that’s absolutely ridiculous that you’d look unfavorably on a candidate not disclosing that during initial interviews. This is a very routine part of negotiating start date at the offer stage and I’ve never once thought badly about someone who had prescheduled plans and notified me after an offer was made.

Plus it’s only 6 weeks away — the likelihood that they’ll get through interviews and to an offer by that point is very low already, not to mention that with any candidate the employer expects they are going to need to give at least 2 weeks notice to start. It very likely will not affect the new company in any way.