as a supplement to the apprenticeship system still in vogue during the seventeenth and eighteenth centuries

—— 总体是个介词短语。as 是介词，as a supplement 表示句子主语 It 的身份，或者作修饰谓语动词的“方式”，不是“时间状语”。

1. to the apprenticeship system：介词短语作supplement 的定语。

2. still in vogue during the seventeenth and eighteenth centuries：介词短语作system 的定语。

3. during the seventeenth and eighteenth centuries：作in vogue（流行）的时间状语。