"Vel" in Latin means "either...or"
Think of it as AKA.
Izrael used LEWIN and WARSZAWSKI interchangeably. Maybe his family was originally from Warsaw and used WARSZAWSKI (a Toponym) before adopting LEWIN as the family proper surname. But both names were used and accepted to identify this family.
JRI-Poland Kielce/Sandomierz/Radom Areas Coordinator