We have some Victorian tufted velvet chairs at my Aunt's place that belonged to several generations of the family. The first thing we'd do to clean them is use a soft upholstery brush attachment on the vacuum cleaner (the round one) and carefully vacuum the entire surface of the chair making sure you get into the tufts. That should get rid of dust and any loose soil on the surface. For stains that have been ground in, I'd initially try dabbing them with just water to see if they come off. Then I'd test a corner of the chair that isn't visible with some upholstery cleaner to determine if the fabric is color safe. If the area you test stays OK then you can use the upholstery cleaner following the directions on the can on other parts of the chair. If there are wood parts, I'd be very careful not to allow any water or upholstery cleaner to get on them as it might damage whatever finish there is. Due to the sun fading you will eventually want to get them reupholstered as sun fading tells me the fabric may eventually fall apart due to sun exposure.