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Neither vertical-tabs-rtl.css from core/misc or from Seven are recognized for RTL. Vertical tabs are not a module, its 2 stylesheets and 1 js file are at the top level of core/misc. The only other stylesheets at that location are print. The 2 stylesheets are overridden in Seven.
Dev tools show no vertical-tabs-rtl.css used while displaying RTL.
Screenshot is showing color red tested in Seven's vertical-tabs.css but not showing an override of color green tested in Seven's vertical-tabs-rtl.css (in addition to not rendering any of the RTL css).