It very much depend on the company. I manage a division at a boutique consultancy. My staff includes IT consultants. Our policy is your vacation time is your own, just give reasonable notice. If we have a really critical project going on, we may ask someone to reschedule. Our policy is non-refundable expenses are re-reimbursed if the vacation is canceled or reschedule at our request. That said, it has happened less than a handful of time and always after amicable discussions with the consultant.

It is not unreasonable for staff to have vacation periods committed many months in advance (weddings, family holiday gatherings). Thanksgiving is a traditional holiday week as many folks travel to be with their families. Companies expect this. If they have minimum staffing requirements for operational reasons, they may have to say no to some employees if everyone requests the same dates. Never happened at my company, but if it did, I would make the decision by the relative seniority of the employees taking into account the dates the employees requested the vacation period.

If I were you, I would notify HR you have a scheduled vacation commitment and ask what is the proper way to work it out in accordance with company policies. If it were my company and I was the hiring manager, HR would pass on the vacation request to me, and I would respond thanks for letting me know, enjoy your vacation.

That said, every company is different. I would expect a much more formal policy at a larger company. How the company reacts will tell you something about your new employer.