Sounds like you are reaching an over-accrual situation.
First the Company Policy should be the guides you use. If they don't have a Policy or Employee Manual, that can be good or bad for both sides.
You are giving them plenty of notice what you want to do, the company ought to be able to work around it. Unless you really are indispensable
.
Apply for it. On your application note that you are going to lose vacation if you don't take it soon. Can they ask you to reschedule? Yes, but it better be for a damn good reason. They really shouldn't (didn't say can't, said shouldn't) not let you go and then tell you you've lost because of max accrual.
Most HR organizations would prefer that you take it all and keep the minimumbalance on the books.
From a financial standpoint un-used vacation is a liability, but HR generally worries about lawsuits and not days of pay.
If they refuse to let you go, and then don't allow you to accrue, you may have a case, but the easy resolution would be to allow you to over-accrue and then make you take vacation at a better time and/or pay you until you are back below max.
HR often forgets that they are there to help the company and the employee, and to help the company make money or at least not spend it when they don't have too.
When I had these things come up, my solution was almost always to let the employee take the vacation or come to a mutually agreeable time frame IF there was a good reason the employee couldn't go when they wanted. And 6 months notice should be fine.
(Then again, when I was in HR or other Administration, we tended to not find out there was a problem until the bel. was ringing. I lose vacation next week and I can't go then. and the Boss is he needs to go but not nextr week can we make an exception NOW?)
I can't think of any good reason that you all can't work it out with this much time.
Vacation is supposed to be both a benefit and incentive, and also to make sure you aren't stressing out or burning out your employees. There isn't anybody so important that they should be in a use it or lose it situation.
From a Financial standpoint the CFO should be happy you are taking vacation, gets the liability off the books. Do you work for him or in his reports?