We are using a process of hand hammering the copper neck tube after it is formed. It serves several purposes: It hardens the metal, making it as strong and pull-down resistant as a brass neck and it also give the neck its specific tonal sound and response characteristics.
 
Sound characteristic:
The Vanguard hand hammered copper has the same mid-tone focused qualities as the M-61 brass. It differs by having slightly less resistance and feeling a little freer blowing. The sound is rich and complex with some added lower mid-tone color, yet retaining the strong resonant projection of the Vanguard line.
 
-From kbsax.com