IDENTITY_INSERT is for temporarily turning off Identity in a given table to allow a value to be specified for that column in an INSERT (otherwise, SQL Server would give an error, and VistaDB has historically just ignored the specified value--except, now, if you use "Strict" compatibility mode VistaDB will throw an error), but it would not reset the seed for when IDENTITY_INSERT is turned back off and Identity resumes. So, it's good to know about for that slightly different use case.
Unfortunately, it's not currently supported in VistaDB. However, it is one of several modern T-SQL language features we are looking to add support for in the near future, probably as part of 5.2 later this year.
You can try in SQL:
SET IDENTITY_INSERT table_name ON
then do your insert
SET IDENTITY_INSERT table_name OFF
Also, if you're using Data Builder you can use the Alter Column GUI to edit the Identity settings.
And there is also an API call IVistaDBTable.CreateIndex(...) to do it from .NET code.
You could try (tested under V22.214.171.1245)
ALTER TABLE ttest ALTER COLUMN ID INT NOT NULL IDENTITY (1, 1);