You might want to clean the siding first. What you see as fading siding MIGHT be a 'chalky' substance. It can be cleaned off with a Vinyl Siding Cleaning Solution at you local HOme Center. Read the bottle first and see if it can help. AND, be sure to clean from the BOTTOM up; NOT the top down. If you try this, do it on, say, the back of the house where it's not seen by everyone.
My best to ya and hope this helps.
Jay J -Moderator
PS: Sure, you can paint vinyl siding but remember, vinyl DOESN'T absorbe paint and vinyl 'moves' with expansion and contraction (I believe) more than wood. Plus, you've just turned a maintenance-free material into one that requires regular maintenance ...