I have been using FMLA for a couple years now to care for my very sick mother. It has always been unpaid and I was fine with that. As of Jan 1st, my employer decided to change this and use up my accrued vacation days instead. I burned through almost all my vacation time this past month taking her for chemo, etc. I don't think they can do this. HELP! I work for a publicly traded communication company.

Yes, they can.

The FMLA statute specifically gives the employer the right to require the use of vacation and sick time during FMLA. This was perfectly legal.