Can vinyl siding be painted or stained?
Yes, you can paint it.  Make sure you clean it very well (use a TSP Cleaner) and then use a primer first (although some sites say you can get away without a primer).  

Also make sure to paint it a lighter color than the current one:


  I assume you're painting with a
  lighter color paint - vinyl siding
  painted a dark color can have problems
  with warping and expansion in direct
  sun.


from http://newsgroups.derkeiler.com/Archive/Alt/alt.home.repair/2008-08/msg03475.html

The expected life of the paint job will be dependent on the weather conditions for that area of the house and probably be about the same as a wood sided area.