A differing voice:
Don't take the vacation unless you can find someone to wholly cover your role well, and you are not the project leader. (caveat... if you prebooked and will lose money or it is a one time vacation like a family reunion, etc.)
Instead, ask to be paid out for it. (if $800 is an issue for you, this could be a nice perk)... OR ask to have it moved into next year. If you combine it with a spring vacation week next year, you now have the option for a long-distance travel, wil less cost because you don't need to move very fast on your vacation (same amount of tourist sightseeing and travel, spread out over 3 weeks instead of 10 days).
Why risk your role / perception at work on a high pressure project in order to take a vacation that you can defer?
Obviously you need to take all your vacations (or be partially paid out, your choice), but taking a longer vacation in March could be awesome, especially if you are the "golden gal" at the office for bringing in a major project success.
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In canada, (and US, I think) employers can tell employees when and how they can take vacation. If work shuts down for 3 weeks at the end of January, and that is everyone's vacation time, then that is when I take vacation, and there is no choice about it. Rescinding approval at the last moment is not called for because employees will leave. Rescinding approval and not allowing vacation within the year is not allowed either, but they could tell you to take the last two weeks of December or pay you out if you opt for the pay out..