Several, completely different formats use .vis extension, so vis to png conversion might actually be many things, but what is certain is that it's related to graphics export to PNG format.
It is most likely searched by people that are looking for a way to export Microsoft Flight Simulator visibility images (.vis) , thinkin these are just standard images. But VIS format from Flight Simulator is actually text based and if any image can be reproduced form the data, it's in the original software only.
Old 3D modeling solution Strata Studio also used .vis files for 3D models, so perhaps export of PNG from these files is possible. The issue will be in finding software that can still open/render content from these .vis files.
Updated: October 13, 2023