Learn Spanish (Latin America): Veíamos el paisaje cuando vino una serpiente. - We were looking at the landscape when a snake came.

This is the classic interrupted action setup! Veíamos (we were looking) is in the imperfect tense because it sets the ongoing background scene. Vino (came) is in the preterite tense because it is a sudden, completed event that interrupted the background action.

I thought ver meant 'to see' and mirar meant 'to look at'. Why use veíamos here?

You are right about the general rule! However, Spanish speakers often use ver for taking in a view, watching a movie, or looking at a landscape. Using mirábamos (we were looking at) would also be completely correct here.

What verb is vino? It looks like the word for wine!

It is spelled exactly like the word for wine, but here it is the preterite (past tense) of the verb venir (to come). Venir is highly irregular in this tense, so for 'it came' (the snake), the stem changes and it becomes vino.

Why does the sentence say vino una serpiente instead of una serpiente vino?

In Spanish, it is very common to put the verb before the subject when a new thing arrives on the scene or is introduced. Una serpiente vino is grammatically correct, but vino una serpiente sounds much more natural for a story.