Random Thoughts Concerning Your Job Offer
Until you get your job offer letter in writing, you do not have a job offer.
Companies aren't in the business of typing up an offer only to have it rejected so by the time the offer is put into writing, they are usually fairly certain that you will accept it.
Otherwise they wouldn't have bothered putting it into writing.
In other words, typically you negotiate the terms of the offer verbally and then once the terms are agreed upon, it is put into writing.
When this process is followed, the time to negotiate terms of your job offer is when you are verbally discussing it, not after your job offer letter has been prepared.
Again, companies aren't in the business of typing up offers to be rejected or amended so if they are smart, they will confirm terms with you before sending you a hard copy.
It doesn't make a lot of sense for them to send you an offer hoping you will accept it and then have to go back and forth typing up amended versions until you are both happy.
Conversely it doesn't make sense for you to verbally agree to your job offer and then suddenly ask for a higher salary or more vacation after you have the hard copy in your hand.
This is typically not a good strategy and could result in the company cancelling your offer if they don't appreciate you asking for a better terms after initially agreeing to it.
Bottom line, until you receive a job offer letter in writing, you haven't really received a job offer.