Why does the airport plate list a VASI that does not exist?
How does an oversight like this exist for such an extended period of time?


Because it doesn't get reported. 


  Is this something that should be reported?


Yes, it should be reported, otherwise this oversight will exist for an extended period of time. 

This dAFD website contains a link that should be followed to [r]eport any A/FD errors or changes. Somewhere down those links you'll find plethora of forms and rather confusing texts which seem not really helpful when you want to report a missing VASI. That brings us back to the answer on your first question.



Update:

In the comments, Kevin Cathcart pointed out the Aeronautical Data Inquiries form which is the appropriate form to flag the problem. Alternatively you can contact the NFDC (National Flight Data Center) via phone at 1-(866) 295-8236.




  Does this create a hazard to potential IFR traffic?


Whether it is a potential hazard depends mainly on the presence of other source of vertical guidance. Is there an ILS / gideslope active? No vertical guidance at all is a safety risk in low visibility conditions.