These two fibres are fundamentally different (silk is a natural fibre, whilst viscose is man-made) and yet display very similar properties.
With Viscose, it is important to never use water or liquids to clean it! Viscose will distort very easily when exposed to water. It is best to follow 'dry-clean only' when it comes to furniture upholstered with viscose content, particualrly when it is high. If the fabric used for upholstery has only a low viscose content, a spot test with a wet fingertip would be advisable in an unseen location on the sofa - to test for recovery as it dries. If it dries normally, light cleaning may ensue. If it does not, dry-cleaning is the best option - though, you might want to try using a soft bristle brush on the patch first to see if this aids recovery.
If dealing with a silk blend, care is also needed for this delicate fabric. Vacuuming is essential to remove loose dirt and dust. A small spot-test of water should be used on the fabric blend in an inconspicuous place (as with viscose), however water and a soft detergent (such as Woolite) should be enough to clean any stains. Do not use a heat-drying method (such as a hair dryer) as this make call shrinking, and be cautious of overwetting. In some instances, marks will still remain after wet cleaning - in which case, dry-cleaning may be preferable also.