on trip中间一般要加冠词a/the,一般表达是“go on a/the trip"。 例句:(1)I will go on a trip to the biggest city in China. (2)When he went on the(这里特指上面这句中的那次旅行) trip (to Shanghai),he did shopping.
on vacation 在度假中,相当于 on holidays
for vacation 为了去旅游,还没在度假。
on 表示正在we are in spain on vacation.(我们在西班牙度假) for表示目的we go to spain for vacation.(为了度假我们去了西班牙)