I assume by three weeks of vacation you mean two weeks vacation and 5 floating holidays. The Vacation time has the half/accrual rule, while the floaters can be used at any time throughout the year they were given. SO if you start early enough in the year to be granted all 5 floating holidays (they are given annually on January 1, and on a sliding scale for hires that start later in the year), from day one, you have 5 Floating holidays you can use, and as one of the above posters mentioned, you can borrow 5 vacation days as well (half of your accrual), so two weeks available from day 1.
If you wait until you actually accrue the 5 vacation days at .84 a month, you can then take all 5 of those, use your 5 floating holidays AND borrow 5 vacation days, putting you out for three weeks, after 6 months with the company. If you need more time than that, it is at your manager's discretion, but will be unpaid.
Having Christmas week off is not up to inVentiv, that's entirely up to the pharma company you're contracted to, some Pharma companies shut down that week, some don't, inventiv never has/does.