这句话是对的,你断章取义了,所以才会引起疑问.补充完整的句子是(She only had )the memory of Mr. Penniman’s flowers of speech, a certain vague aroma of which hovered about her own conversation.这是个定语从句.我们来分析句子成分,先行词是flowers of speech“华丽的词藻”,后半句用了比喻的修辞手法,说那些华丽语言的余味还在萦绕,也就是说vague aroma 是flowers of speech的.所以必须用of which,which代指flowers of speech.
如果你认为完整句子是a certain vague aroma of which hovered about her own conversation.这样的话,你的疑问就产生了,这样你的想法也就对了.