Is there a difference between visual short term memory and visual working memory? As far as I can tell the two terms are used interchangeably.  Do these two separate terms exist for historical reasons, or is there a distinction I'm missing?
The distinction between these terms is a matter of ongoing empirical research.  They form part of a set of potential taxonomical categorizations of visual memory (VM).  So far proposed sub-parts of VM include: Iconic (or sensory) memory, short-term (VSTM), fragile short-term (fVSTM), working memory (VWM), and long-term (VLTM) memory.  The distinction between short-term and long-term visual memory is generally accepted, as is iconic memory; the others are still actively debated.

As the neurological validity of these terms is still controversial, it is not unusual for them to be used interchangeably.  For example:


  We consider VSTM and visual working memory (VWM) as the same set of
  processes.


The additional sub-part distinctions are proposed by Sligte and Lamme (2011) (who refer to VSTM as an umbrella term for all the non-long-term memory sub-parts):


  Traditionally, VSTM is thought to operate on either a short time-scale
  with high capacity - iconic memory - or a long time scale with small
  capacity - visual working memory. Recent research suggests that in
  addition, an intermediate stage of memory in between iconic memory and
  visual working memory exists. This intermediate stage has a large
  capacity and a lifetime of several seconds, but is easily overwritten
  by new stimulation. We therefore termed it fragile VSTM.