Need help with a pre employment vacation letter...

I started in November. Wanting to take a vacation in April that I have been planning on going on with a buddy for a year now. The training period is 6 months long at least. I'll have 80 hours of vacation time banked by then but I'll only be using 40. I ask my manager if its ok to request that time off even though I'm still in the training phase. He was fine with it and just wanted me to write a letter saying we discussed it prior to employment and to date it for before my start date just in case anyone says anything. I'm sitting down to write said letter and I don't know how I'm suppose to write it or what its suppose to say.... Any help?

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