Her genealogical sketch is here.
From Wikimedia; linear flaws and some spots removed with Photoshop.
Comtesse Antoinette Élisabeth wears a blue velvet (?) jacket with a décolleté square neckline over something that provides a bertha. A scarf fills in the décolletage. Her jacket features very wide lapels and is decorated with bows instead of buttons.
Keywords: 1785, Vigée-Lebrun, Antoinette Elisabeth d'Aguesseau, Comtesse Louis-Philippe de Segur, comtesse de Ségur, d'Aguesseau family, de Ségur, Countess, French, long curly bouffant coiffure, feathered ribboned hat, earrings, neckerchief, jacket, square décolletage, lapels, lace bertha, bows, long tight sleeves, lace back-flared cuffs, peplum, full skirt