His letters, in French, sometimes of great length (e.g. 12 Aug. 1956 - 25 pages), are full of details of his daily life and work, meetings with friends and acquaintances, travels, exhibitions of his work, his health, and his feelings for Gillian Sutro, with whom he had a lifelong affair. Meetings include for example accounts of Françoise Sagan, 1955 (MS. Eng. c. 7238, fols. 173v-175r), and Georges Simenon, 1954, 1956 (MSS. Eng. c. 7238, fols. 23v-27r, 32v-33v, 35; c. 7240, fols. 5r-7r; with a pen-and-ink portrait of Simenon, 1954, MS. Eng. c. 7238, fol. 33r). Many of the letters are dated by Gillian (from the postmark, not the day of writing), and many of the envelopes have annotations by her, in English (about the contents of the letters, and her own life at the time); with some later notes and cuttings (e.g. obituaries of persons mentioned) added by her.

Drawings or small (some of them very small) sketches by Vertès are MS. Eng. c. 7235, fols. 2-3, 34v-35r , 90v , 91v, 92v (copy on display in the Sutro Room, Trinity College), 190; MS. Eng. c. 7236, fols. 26, 163v; MS. Eng. c. 7237, fols. 84, 120, 315v; MS. Eng. c. 7238, fols. 30, 33, 45-6, 52, 137, 139; MS. Eng. c. 7239, fols. 7v, 44, 159, 174v, 201v, 231v; MS. Eng. c. 7240, fols. 13, 112, 176v, 186v, 303, 307; MS. Eng. c. 7241, fols. 29, 237v, 251, 293, 295v, 329, 332; MS. Eng. c. 7242, fols. 2r, 2v, 74, 146v, 163, 214, 266. A few have been replaced by photocopies (the originals being restricted for preservation).