In 1853, an atlas of historical plans and maps of the Neva delta — the area where in 1703 Peter I founded the northern capital — was published in St. Petersburg for the city's 150th anniversary. According to the explanation to the maps, the island marked on the 1700 map with the letter H is Hirvi-saari. Losiy (now Vasilievsky)

The value of this information, besides the Finnish toponym, is also that the 1853 atlas recorded one of the earliest uses of the modern form of the Russian name — "Vasilievsky Island." The timeframe for the existence of the simplified form, Vasiliev Island, as well as its replacement by the extended form (with the ending -sky) is not a toponymic but a historical-linguistic question: the root of the toponym is the same here.

At the same time, there was another toponym — Dammargolm, the Swedish name of the island. It is unclear whether the toponym Dammargolm refers to 1737 or if it existed already in 1698, when the first original was created.

On the 1717 map, the island is listed as named after a certain St. Vasily. All toponyms are given in French: Isle de S^t. Basile. There are several saints named Vasily in Christianity. Most likely, the mapmaker, not knowing which Vasily the island was named after, arbitrarily added "saint."

Considering the Russian tradition and the careful attitude of Orthodox Christians to sacred matters, if the island had been named by Russians, it would not have been called Vasilievsky but Svyatovasilievsky (Holy Vasily’s).

The 1720 map, compiled by J.B. Homann, records the toponym in German: Wasili Osterow. This shows that the original Russian name of the island was more likely "Vasiliev(sky) Island" rather than "Island of Vasily" — otherwise it would have been written Osterow Wasilia.

There are several legend-versions of the origin of the name:

The geographical dictionary published in the 1870s by Petr Semenov-Tyan-Shansky gives the following explanation: The largest of the islands in the entire Neva delta is Vasilievsky, called during Swedish rule Irvisari, or "Elk Island," and received its current name from the officer Vasily Vasily Korchmin, to whom Peter I sent orders with the laconic inscription: "To Vasily on the island."

Until 1837, the opinion of a late — Peter the Great era — origin of the name Vasiliev Island was generally accepted, and Vasily Korchmin, supposedly giving the island its name, really existed and commanded an artillery battery stationed at the Spit since 1703, exactly where the Rostral Columns now stand.

In 1837, an article by an anonymous author appeared in the journal "Son of the Fatherland", stating that Vasiliev Island was called so even before Peter the Great’s time. In published documents, mentions of Vasiliev Island date back to 1545, i.e., the name must have arisen no later than the era of Ivan the Terrible. This note provoked a strong rebuttal from the then well-known historian, full member of the St. Petersburg Academy of Sciences, P.G. Butkov.

He argues that the Vasiliev Island mentioned in the 1545 charter is a different one, not the one located at the mouth of the Neva. Butkov writes: "So, let us leave the name of the Petersburg Vasiliev Island to Korchmin, by right of many antiquities, until another Vasily appears among Peter the Great’s associates, who can present indisputable evidence to the court of sound criticism for claiming this honor." Thus, Butkov supported Vasily Korchmin as the godfather of Vasiliev Island.

An argument in this historian’s reasoning for distinguishing the Vasiliev Island of the 1545 charter from the Vasiliev Island at the mouth of the Neva is that the charter lists two bridges side by side: the Kleti row in the Izhora parish and Vasiliev Island. Therefore, he believed that Vasiliev Island lies somewhere far from the mouth of the Neva. Unfortunately, he could not have seen the second half of the cadastral rent book of the Votskaya pyatina of 1500, published only 11 years after his article appeared. Had he looked into this cadastral book, he would have seen the following: it places Vasiliev Island next to "the great prince’s row at Kleti on the Izhora river," exactly as in the 1545 charter, despite the fact that the "row at Kleti" is quite far from Vasiliev Island and is located in a different parish (Izhora, not Spassky Gorodensky) — "seven versts from the Neva," as the cadastral book says. Moreover, the book explicitly states that Vasiliev Island is located "at the fish mouth on the Neva," thus removing all doubts.

There is also a known letter dated July 2, 1426 from the Narva magistrate to the authorities of the city of Reval, where a Russian named Sava (Sabe) from Vasiliev Island (Wassilighenholm), which is on the Neva (in der Nu), is mentioned. From Middle Low German, the word "wassilighenholm" can be translated both as "Vasiliev Hill" and as "Vasiliev Island".
Further, in 1851–1852, the Moscow Society of History and Antiquities of Russia published the "Cadastral rent book for Novgorod of the Vodskaya pyatina 7008 (1500) year (second half)." This invaluable historical document contains a detailed description of "Vasiliev Island at the mouth of the Neva," from which it is clear that the name Vasiliev Island existed even in the 16th century, i.e., hundreds of years before Peter the Great and Vasily Korchmin.

At the end of the 15th century, according to the mentioned cadastral book, there were 17 arable fishermen and 18 non-arable fishermen on Vasiliev Island. During the Swedish period (17th century), Vasiliev Island had three names simultaneously — Russian, Finno-Ugric, and Swedish. The Russian name remained the same: Vasiliev Island (in the Swedish cadastral book — Wassilie Ostroff); the Finnish Hirvisaari — Elk Island (in the Swedish cadastral book — Hirfwisari); the Swedish Dammarholm, meaning Pond Island, because it had fish traps belonging at the end of the Swedish period to the well-known Delagardi family. On Vasiliev Island during Swedish times, there were three settlements: on the Spit, on the Smolenka river, and at the northern tip of the island by the sea.

It should be considered that Vasily, who gave the island its name, was not an Izhora (although both Izhora and Vot were Orthodox and bore Orthodox names, unlike the Finnish Ingermanlanders), but a Russian (Novgorodian) who most likely lived in the 14th or early 15th century. It is unknown whether he became the owner of the island or was simply the first settler. He could not have been any of the Novgorod posadniks of the third quarter of the 15th century named Vasily.

Vasiliev Island was at risk of being renamed from above during Peter I’s reign. For some time, it was officially called Preobrazhensky. However, this name did not take root.

Source: https://ru.wikipedia.org/wiki/История_названия_Васильевского_острова