Is it too late for me to negotiate my salary?

Well first off your math logic is a little off. You state that instead of getting the full 6 weeks of paid vacation you'll only be getting 4.5 weeks the first year and because of that you'll be earning $1000 less the first year. That's not true. You're only losing 1.5 weeks of paid time off. But you're still getting paid....you just have to work those days instead of sitting at home. Look at it this way, if the salary is 37500 a year, with or without vacation, you'll still be earning 37500 a year. The only difference is that on some days you can stay at home at watch TV and still get 37500/year.

Now on the subject of the time you're taking off at Christmas that will be unpaid. That's just something you'll have to talk to them about. But keep in mind that you'll only be working for less than 2 months before you start taking time off. It's a little un-realistic in my opinion for you to work for 2 months and then expect to get a paid vacation so soon. I don't know of too many companies that would allow you to have a paid vacation after such a short time of working. Most companies require you to work a full year before you get any paid time off.

Sounds to me like they are being more than fair. First, they didn't try to low-ball you on the salary and offered you a salary higher than the low end of your range. Second, they give 6 weeks paid vacation! That's unheard of for most companies. In most companies, people have to work 10-15 years before they're eligible for that much vacation. Most places only give 2 weeks vacation for new hires. To give 6 weeks after the first year is awesome. And finally, the fact they are willing to let you take any time off (paid or not) after only 2 months of work is pretty understanding of them.

You haven't signed anything, so it's isn't too late. But keep in mind that the potential employer will have the option to withdrawal the offer when you try to negotiate. Chances are, they won't do that, but it's a risk you take. Most employers won't allow a new employee to take a paid vacation after working for a month or two, so that should have been expected. Keep in mind that if the benefits at this job (including health insurance, retirement, etc.) are better than at your current job, you might actually make more money, even though you may see less of it in your pocket.

But the salary is accurate. Your base salary is $37,500. Beginning August 2012, you'll be paid $37,500 annually. You can't expect to get paid a full year's salary and get a full year of paid vacation if you don't work the full year. And like the other person said, $1000 or even $2000 isn't a big deal. If you're a good employee, you'll get a raise in no time.

You appear to have unrealistic expectations of what this employer will do for you. You should not make this an issue if you need a job. The job offer can easily be withdrawn. I'm sure that they will not have a problem finding someone else who is perfectly happy with the offer. Earning $1000 less in the first year should not be a problem.