Couldn't it be that
Silas was a short form of the Latin
Silvanus in first century Greek, and that it was used by Jews of the time as a Greek equivalent of
Saul? There is a man mentioned in Acts and Romans in the New Testament called
Jason.
Jason was obviously a Greek name, but I remember reading somewhere that it was used by Jews during the
Roman empire as an equivalent to
Joshua.
There are plenty of examples of contact between two cultures speaking different languages where certain names in one language are accepted as the "equivalent" of names in the other language even though there is no etymological connection.
Cornelius, for example, was formerly used in
Ireland as the "equivalent" of
Conchobhar.
Anyway, things do get complicated and hard to interpret when different languages are in frequent contact with each other. :)