I had two quests, one for legendary item the one where you need vessel two missives and soul ashe. Then I had the tailoring quest for I dont know soul ashe or something for runecarver anyway they were both complete and ready to turn in so I made my legendary first and my tailoring quest disappeared and now I have no option to upgrade to level 5. Do I have to do korthia quests or whats the deal? in my log it says I still have to assisst the runecarver but there arent any quests available.
The Vessels of the Thread quest starts with the Runecarver (after you complete The Final Pieces ).
You then take the Memory to the Tailoring Trainer in Oribos (Stitcher Au’phes).
More info on Tailoring legendary armor: https://www.wowhead.com/guides/shadowlands-tailoring-profession#legendary-set
That quest gives you the initial Grim-Veiled recipes. Additional ranks for each recipe are learned via a legendary experience path.
Link: https://www.wowhead.com/guides/shadowlands-tailoring-profession#legendary-crafting-xp
Rank 5 and Rank 6 items are created by combining Rank 3 and 4 items with the optional reagent 
Vestige of Origins.
Link: https://www.wowhead.com/guides/shadowlands-tailoring-profession#optional-reagents