Hello and welcome to JustAnswer.
I'm sorry to hear about your situation and hope I can help.
Yes, your denial of your use of vacation time in order to accomodate your disability could definitely be seen as you being denied a reasonable accomodation.
You should file a complaint with the Department of Fair Employment and Housing for discrimination in violation of the Fair Employment and Housing Act.
You can do so using the instructions available online here:http://www.dfeh.ca.gov/Complaints_eCompProc.htm
Unfortunately, unless your termination was due to discrimination due to your disability, it would be considered legal.
As an at-will employee, you can be terminated at any time for any reason with or without any prior notice.
This stems from the employment at-will doctrine, which is codified in California Labor Code Section 2922, and states:
"An employment, having no specified term, may be terminated at the will of either party on notice to the other. Employment for a specified term means an employment for a period greater than one month."
I wish I had better news to give you, but I hope you appreciate a direct and honest answer to your question.