No, it wasn't. Rio is just used here as a faraway and almost unreachable destination. Brasilia (Brazil) had its share of Russian immigrants, but in 1931 it was very hard and extremely expensive for the Soviet citizen to get there.

And you forgot the white pants, Ostap Bender wanted to wear there. It sounds like a quite strange and specific request, right?

There are several theories why Ostap wanted to live in Rio, wearing his white pants. But the most believable, in my opinion, is like that: this is an hommage to Michael Bulgakov's play Zoya's apartment, 1926. This play was very popular right before the first novel about Ostap Bender was published (12 chairs, 1927). And one if its characters, Ametistov, says that he wants to see Nice (France), Azure coast, and he wants to stay on this coast in his white pants.

Nice was a very well-known resort for Russian readers. It was a must-go destination for Russian aristocrats since 1850. Russian tsars and their court had their villas there. There is a La Cathédrale orthodoxe russe Saint-Nicolas that was built by Nicholas II. So, if you don't like new Soviet Russia, and you have money, this is the most obvious destination you are having in mind.

Why it was changed by Ilf and Petrov to Rio, nobody knows, but Rio — let me repeat that — sounds extremely unreachable and absurd.

This theory cannot be proven or disproven, we aren't able to scan the writers' minds, but there are several other arguments that Ostap Bender was in part based on Ametistov. Like Bender, Ametistov is a fraud. Like Bender, Ametistov has several names for different uses. And he has a sidekick, and his surname is Obolyaninov (compare to Kisa Vorobyaninov, Bender's sidekick from 12 chairs).

So, now, it didn't have a special significance, and it wasn't perceived as today's Las Vegas, but some could say that this changed after the book, thanks to great popularity of these two novels and movies that were based on them.

Source: 12 chairs and Zoya's apartment (sorry, it's in Russian).