I believe that Stephen Vost Janssen is the nephew of my GG Grandfather Thomas Vost. Stephen came to Australia as a child with his mother Margaret and aunt Jane in 1885. Margaret married Charles Janssen in 1887 and I am not sure if the Stephen was formally adopted but he took Charles' name. Stephen was an accomplished violinist.
In 2001 the Glebe Music Festival performed the program to celebrate the federation of Australia. The program was the same as performed on 25 June 1900 to celebrate the birthday of Queen Victoria.
I know very little of Stephen Vost Janssen’s life except the basic details. He was born in Newcastle-Upon-Tyne, England in 1880. His mother is listed as Margaret. Stephen married Marion Hamilton in Sydney, Australia in 1914 and died in 1945 also in Sydney.
I hope this information helps your search. I will attempt to track down a photo of Stephen in the collection I have or an elderly relative has so that you may compare.
I would be interested in where this search takes you.