I am taking vacation time for my child's birth, but my work wants me to fill out FMLA forms. Do I have to?

I have requested two weeks of paid vacation I have accrued and it was approved. However, when my work found it was because my wife is going to have our baby then and I wanted to spend the time at home with both of them, they wanted me to fill out FMLA forms. Do I have to fill these forms out? If my vacation time has already been approved should it make any difference why? I don't plan on taking any more time off other than what was requested. Thanks.