For vocabulary acquisition I believe natural acquisition is more useful -- learning through context. Like when my Spanish conversation partner taught me la peluquería and la barbería, I'm pretty likely to remember them because of how much trouble I had saying peluquería.
Indeed, books are the best for natural vocabulary acquisition -- in the same we learned vocabulary in our native language, through context. All the more so on Kindle, where you can translate any word with a press on the screen. Spanish is a language where reading in Spanish (subtitles or books) is super useful because the pronunciation is 100% predictable.
The type of vocabulary Duolingo teaches tends to be something you'd pick up quickly anyway, and I found its gamification is less effective than spaced-repetition through Anki (which is free and has free flashcard decks) or 200 Words a Day (which costs money but has 2000 mnemonics).
You may also enjoy the Complete Spanish - Language Transfer podcast, which will instantly give you a vocabulary of 2000-4000 words by teaching you common roots (like comunicacíon, comunicar): https://www.youtube.com/watch?v=6OZbL91GXGI&list=PLeA5t3dWTWvvwf5fw0Nl7mVk0OUjP1Ln2 (free on YouTube)
Similarly I originally learned Spanish from the Michel Thomas method's beginner course. It's amazing to go from nothing to conversational in 8 hours with decent pronunciation. The whole bundle is $273.
To be perfectly honest, I learn very little from my conversation partners or PT patients at work, because they never correct me. (It breaks up the flow of a conversation too much, and people don't want to "be mean.") My weekly chats with conversation partners are a nice way to enjoy some socialization and to practice production and comprehension, but it's low-reward for the effort compared to the other strategies I mentioned.
After all, how many people have you met who have learned English "through immersion" without formal classes but have significant errors in their ability to communicate like a native speaker, even after 10 or 20 years living in the US and working with English speakers?
The other problem with learning without a teacher is that you can develop a terrible accent, and a professional teacher is usually the only one who will take the time to correct you. Spanish isn't difficult in terms of its sounds, but it is a stressed language, and Americans tend to mess up the stress.
Finally two books I've really enjoyed are Spanish In Your Face (for emotional vocabulary to describe people) and Correct Your Spanish Blunders.
As you can guess, these are both very difficult types of information to get anywhere else, but they're critical for fluency.