Is this him? Also, it appears his 1912 service file (etc) is on Ancestry.
UK, British Army World War I Medal Rolls Index Cards, 1914-1920
Name: Arthur Robt Vincent
Military Date: 1914-1920
Regiment or Corps: Gloucestershire Regiment, Somerset Light Infantry
Regimental Number: 1589
UK, British Army World War I Pension Records 1914-1920
Name: Arthur Robert Vincent [Arthur Roberts]
Birth Date: abt 1876
Birth Place: St Georges, Gloucestershire, England
Marriage Date: 25 Sep 1905
Marriage Place: Registrars Office
Age: 36
Document Year: 1912
Regimental Number: 1589
Regiment Name: Gloucestershire Regiment
Form Title: Army Reserve (Special Reservists) Attestation
Number of Images: 26