[zork]

Thanks. The missing link was GetAtlasInfo() which is not described anywhere.

It is working now.

local function OnVignetteAdded(self,event,id)
  if not id then return end
  self.vignettes = self.vignettes or {}
  if self.vignettes[id] then return end
  local vignetteInfo = C_VignetteInfo.GetVignetteInfo(id)
  if not vignetteInfo then return end
  local _, width, height, txLeft, txRight, txTop, txBottom = GetAtlasInfo(vignetteInfo.atlasName)
  PlaySoundFile("Sound\\Interface\\RaidWarning.ogg")
  local str = "|TInterface\\MINIMAP\\ObjectIconsAtlas:0:0:0:0:256:256:"..(txLeft*256)..":"..(txRight*256)..":"..(txTop*256)..":"..(txBottom*256).."|t"
  RaidNotice_AddMessage(RaidWarningFrame, str.." "..vignetteInfo.name.." spotted!", ChatTypeInfo["RAID_WARNING"])
  print(str.." "..vignetteInfo.name,"spotted!")
  self.vignettes[id] = true
end
 
-----------------------------
-- Init
-----------------------------
 
--eventHandler
local eventHandler = CreateFrame("Frame")
eventHandler:RegisterEvent("VIGNETTE_MINIMAP_UPDATED")
eventHandler:SetScript("OnEvent", OnVignetteAdded)

Btw here is sth I found in the Blizzard_TutorialLogic.lua. It is a atlas to string texture converter if I see that right. It might be able to upgrade my current solution.

function TutorialHelper:FormatAtlasString(str)
    return (string.gsub(str, "{Atlas|([%w_]+):?(%d*)}", function(atlasName, size)
                size = tonumber(size) or 0;
 
                local filename, width, height, txLeft, txRight, txTop, txBottom = GetAtlasInfo(atlasName);
 
                if (not filename) then return; end
 
                local atlasWidth = width / (txRight - txLeft);
                local atlasHeight = height / (txBottom - txTop);
 
                local pxLeft    = atlasWidth    * txLeft;
                local pxRight   = atlasWidth    * txRight;
                local pxTop     = atlasHeight   * txTop;
                local pxBottom  = atlasHeight   * txBottom;
 
                return string.format("|T%s:%d:%d:0:0:%d:%d:%d:%d:%d:%d|t", filename, size, size, atlasWidth, atlasHeight, pxLeft, pxRight, pxTop, pxBottom);
            end));
end

Nice that actually was helpful. Instead of the texture path hard coded you can use the texture file id instead. Got it working now.