Carpet is similar to natural fiber materials, but there are a few key differences.
Artificial fibers have a Diffuse color that is different than Black. Use a Falloff map to create an illusion of slightly fuzzy edges.
A very common effect is that the ends of the fibers are soft and fuzzy. A similar look can be faked by using VrayHairInfoTx in the Transmission slot and setting the ends to have white transmission color. The bias should be adjusted so that only the very tips are affected. For this effect to look good, you need to lower transmission glossiness to about 0.5.
If you want to use a texture for the carpet, it should go into the Diffuse Falloff slots and the Transmission color map. Adjust the mix and colors by eye until you get a good looking result. Try mixing the second falloff (and hairinfotx) slot with white or light gray and tweak the percentage. Of course, using bitmaps is perfectly acceptable as well.