Arnold has an anisotropy slot in the standard shader that takes a float between 0 and 1 for rotation, so you would need to convert the vector representation to a scalar. I assume this is an 8 bit map, with red being x and green being y, negative values in 0-127 and positive values in 128-255. Subtract 0.5 from each channel using aiSubtract. Plug red into the X input of an aiAtan node, green into the Y input. Choose to output in degrees and divide the result by 360 before plugging into the anisotropic rotation of the shader. If the texture is also meant to yield anisotropic magnitude, you can use the aiLength node on the output of aiSubtract.