on their vacation
和
for their vacation
都对,但意思不同
on their vacation 度假时,度假期中
for their vacation 度假形式,度假方法
My parents met some old friends on their vacation
度假期中我父母遇见一些老朋友
My parents went to New York for their vacation
我的父母去纽约度假
不同语境,不同介词
希望帮到了你,