Mr. Vitkher's wife was Sophia Annetta, last name unknown for now, who was born in what is now Germany around 1731. His birth year is based on Sophia Annetta's birth year, extrapolated from her age in the 1767 Grimm census. Mr. Vitkher married Sophia Annetta about 1754. The couple had two children, Ludwig and Sophia. Their last name at birth was omitted in the 1767 Grimm census but confirmed in the 1775 Grimm census.
I checked all the Russian immigration sources for people named Vitkher, and there were none. I reviewed many different possible spellings of the name, without finding a match.
By the time of the 1767 census, Sophia Annetta was married to Gottlieb Schultz and the couple had a two-week-old baby. This implies that her first husband didn't survive long after arriving in Saint Petersburg, and she remarried while the immigrants were still in temporary quarters during the fall and winter months. She gave birth to daughter Maria Katharina Schultz two weeks before the census was taken a few months later.
Head of the Household: Gottlieb Schultz, 27, Lutheran craftsman from Darmstadt
Wife: Sophia Annette, 37
Stepson: Ludwig, 12 [no surname given]
Daughter: Sophia, 7 [probably also a stepdaughter]
Daughter: Maria Katharina, 2 weeks old
The 1775 census confirms that daughter Sophia was not Gottlieb Schultz's biological child, as well as his stepchildren's surname: Vitkher, most likely a Russian interpretation of the name Victor, Whittaker, or something that sounds similar. (In German, the letter W is pronounced like a V.)
Eichhorn, Alexander, Dr., and Dr. Jacob and Mary Eichhorn. The Immigration of German Colonists to Denmark and Their Subsequent Emigration to Russia in the Years 1759-1766, Druck and Bindung: Druckerei and Verlap Steinmeier GmbH & Co. KG, 86738, Deiningen, Germany; Published 2012.
Pleve, Igor. List of Colonists to Russia in 1766, "Reports by Ivan Kulberg," Ministry of Education and Science of Russian Federation, Saratov State Technical University; Published in Saratov, Russia 2010.
Rauschenbach, Georg. Deutsche Kolonisten auf dem Weg von St. Petersburg nach Saratow, Transportlisten von 1766-1767, Published in Moscow, Russia, 2017.
↑Transport of the Volga Germans from Oranienbaum to the Colonies on the Volga 1766-1767. Translated and edited by Brent Alan Mai; Published by the American Historical Society of Germans from Russia, Lincoln, Nebraska, 1998.
Pleve, Igor. Einwanderung in das Wolgagebiet 1764-1767, Band 2, Herausgegeben von Alfred Eisfeld under Mitarbeit von Sabine Eichwald, Published by the Nordost-Instsitut - 38085 Göttingen, 2005; page 76, family #26.
↑The 1775 and 1798 Census of the German Colony on the Volga, Lesnoy Karamysh, also known as Grimm; Published by the American Historical Society of Germans from Russia, Lincoln, NE, USA; Published 1995, family #110.