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