After I graduated from college, I used to start my vacation planning process by talking with my boss. I would propose a date and duration -- in essence asking permission to take vacation. Then the boss would veto or approve it and I could buy tickets, hotel rooms, etc.
This approach to vacation had several flaws. First, I behaved like vacation was a privilege instead of a right. I strongly believe that vacation is a right and a necessity. Vacation hours are not a favor or a bonus -- they are part of your compensation.
Second, it made changing my plans complicated -- if I couldn't get tickets for those dates, did I have to go through the process again? The extra step also makes it harder to get a good deal on hotels or travel. Friction of this sort discourages vacation rather than encouraging it.
Now that I'm slightly wiser, I use ideas I stole from negotiation tactics. I start with the Fait Accompli. Instead of asking and then planning, I buy all the tickets and hotel reservations first. Only then do I tell my boss the dates I'll be gone.
This way, I've put myself in a strong position. If the boss objects to my choices, his recourse is limited. If the dates are bad, he gets to worry instead of me. If the boss demands I stay, he looks unreasonable. Am I supposed to just abandon all the money I put into the plans already?
Even if the boss insists you stay, it will be perfectly reasonable for you to demand reimbursement for your losses. Just the prospect of trying to expense such an absurd thing -- the cost of ruining your vacation plans -- may change your boss's mind.
The key to this tactic is to act assertively but always appear in control and reasonable. I'm not saying you shouldn't act surprised if your boss objects -- a good negotiator would. But you should always be in control so that any objections sound unreasonable.
Another tactic is to always announce your vacation two or more weeks in advance if possible. Two weeks has the advantage of a strong association with the accepted notice when quitting. If you really felt strongly about taking this vacation, you have the (risky) option of calling your boss's bluff. "You say three weeks notice isn't adequate, but if I were quitting this would be a week more than the standard two weeks notice."
Equally important, you must always carefully communicate your vacation plans. If you use some sort of online vacation system, immediately put the dates into it. Send your boss and coworkers an email too. Don't act passive-aggressively. If anyone has objections you want them to come out immediately or never.
Planning vacation this way also helps to train your boss and coworkers. He or she will start to realize that there are consequences for poor planning and communications. It will become obvious that last minute crises and under-staffing can't work when engineers actually use their vacation. It also provides a useful reminder to both the boss and you that the boss is not a master, but a supervisor.
Keep in mind that these tactics can hold risks for you. Your boss may not be used to engineers taking charge of their own lives. If so, this is one way to start getting him used to the idea. It's also possible, although I think unlikely, that you may end up faced with the choice of losing the money you spent on tickets or taking vacation without your boss's blessing.
Despite the risks, I think defending your vacation is important. Although there are certainly some engineering jobs out there which are adventures in themselves, I think a balanced life requires an escape from the work environment. Unpaid activities are often the most memorable and rewarding.
If you want to learn more about negotiation, I suggest the book Secrets of Power Negotiating. You can find more information about this book in my recommended reading list for Engineers.