Vacations during my one year postdoc
Anywhere in Europe, you are entitled to paid vacation. How much vacation you get depends on the country and job, but if online resources are to be trusted employers in France should be fairly generous with about 5 weeks of paid vacation per year (note that this is not outrageously high - most countries in central Europe give you between 4 and 5, sometimes up to 6 weeks of paid vacation).

That you are there "only one year" does not matter. Even if you were employed only for three months you would be entitled to paid vacation time pro rata (e.g., about 1.25 weeks of paid time off for a 3-month internship), although for such short employments it is common to not take all vacation and instead get monetary compensation.

One thing to consider is that there is no general right to take all of this vacation in one piece, or at any time that is convenient to you. That is, you should still plan that your mentor may request you to take your vacation primarily during the summer or winter holidays.