You should mention in the online help text at https://helpx.adobe.com/indesign/using/frames-objects.html#object_export_options_cc_cs6_cs5_5 that a vector object's alt text will not be included in the epub file unless you also set the export settings to rasterize the object. (Basically: you can't include a vector object to an epub file in a vector format. Not even it's alt text.)