convert VHDL to C
I think you are confused! You probably don't need such a converter.
VHDL is for designing digital logic. The Xilinx ISE tools synthesize your VHDL and download it into the FPGA.
MicroBlaze is a microprocessor core (written in VHDL, but encrypted because it's proprietary) that you can also synthesize and put into the FPGA. MicroBlaze, like other microprocessors, executes its own unique machine code. You can generate MicroBlaze machine code by compiling your C code with the GNU C Compiler (GCC) that's included with the Xilinx EDK software package. Then you store the MicroBlaze machine code in a ROM or RAM that's connected to the MicroBlaze's instruction bus.