? on written/unwritten agreements

Hi, I know I am one of the last couple people for an opening; just awaiting final word. I need off for a week for a planned trip/sibling's wedding in a few months. If offered the job, I need to bring it up. From what I was told in terms of my vacation days by that point, I will be short one day.

So, Can I possibly do something unpaid or get an extra day if possible negotiated it into my offer?
And if I get some sort of agreement where I am off that week; Is that just verbally or can I get it in writing? I've never had to ask for something like this; but I know people have negotiated things like this before upon hire.
thanks

Accept the job and then tell your manager. I have taken time off w/o pay w/a large pharma company and also on a contract. Don't know when it is, but PDI is closed the week b/w Christmas and New Year's.

Do not take this advice. You will be fired if you do.