"They are vastly different" 是表示两者截然不同的常用表达方式。
"Vastly" 是一个形容词，意思是 "非常"、"极大地"。
"Different" 是形容词，意思是 "不同的"。
举例：
The two paintings are vastly different in style. (这两幅画的风格截然不同。)
Their personapties are vastly different. (他们的个性截然不同。)
The two books are vastly different in terms of content. (这两本书的内容截然不同。)