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.
Working memory is a term introduced in 1960 in the context of cognitive planning. It is in one sense a short-term memory, a temporary storage for short-lived information. The distinction lies in "working", which implies that the contents in memory are being worked upon. (A physicist would define work as the act of a force that leads to displacement.) Thus, when we design experiments to measure working memory performance an additional requirement is that the memory is updated. For example, the participant has to keep an item in memory, discard it, and replace it with a different item. The participants often have to evaluate when to update their working memory contents, depending on the task.
Maintenance and recall of visual stimuli is implied in both terms. However, "working" memory further implies that the item or items being maintained change over time.
This is the case in human cognitive neuroscience. However, in the animal literature "working memory" can mean something else entirely. In the end, it is probably more important for memory researchers to ask whether these are different concepts than for researchers in visual perception and psychophysics, so I would avoid drawing conclusions based on the view of the visual research community alone.