Since when "vacation" is a verb?
English is an action language; many, many nouns can be used as verbs.
You speak about american english, don.' t you?
While that might be true, I've literally never heard someone use "vacation" as a verb and I would think people will be slightly confused once you do.
Maybe it's regional then, because here in my part of Southern California we say it often. "I vacationed in Hawaii." "She vacations in Florida every year."
I'm on the other side and people use vacation as a verb here too
The term "action language" is actually a thing? As in used in linguistics?
why not "함께"?
여행을 같이 가자 this should be correct right?
There is a saying: "In English any noun can be verbed". The saying is funny because it shows it is not quite true.
Why cant we say "함깨 여행합시다" ? Does it mean let's go vacation together?
You can, though Duolingo here is looking for the sentence at the Causal speech level.
Would 이랑 work or must that be used in conjunction with a noun?
(이)랑 requires a noun. It is essentially the intimate version of 하고 and 와/과.