Yes, when you're saying that "you are going to [do something]" you need "vamos a + infinitive".
But in this lesson we are explaining how to conjugate the verb ir in the present tense (simple present tense, I do, I work, I say, I eat...) though, not the near future "I'm going to do something"; this would be "Vamos a viajar a Lima".
There is an expression in Spanish "ir de viaje" which means to "go travelling". This is what we're using here "Vamos de viaje a Lima". I know that having a translation in English using the continuous tense "I am going travelling..." creates a bit of confusion, but this is the translation used because it would sound a bit odd to use the simple present "I go travelling in Lima".
The present tense in Spanish often reflects an action that a person "is doing" more than "does". I imagine this is where the confusion is.
Here is a Kwiziq lesson that is explaining the near future tense using "going to do something" and where you will see the structure using the infinitive of the main verb.
I hope it clarifies it.
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