Vocabulary and fluency
How large of a Spanish vocabulary is necessary to reach the minimal level of fluency?
Here I count all conjugated forms of a verb as a single word--including the gerund and past particple, even when used as an adjective.
Adverbs are only counted as new words when they don't come from ading "mente" to an adjective.
Nouns that are a conjugated form of a verb are counted as new words, so gobernar and gobierno are separate words.
Multiple definitions for a single word do not count as new words, nor do compound forms and idiomatic phrases, although clearly they would have to be understood in order to be fluent.
I would guess around 3000 verbs, 6000 nouns, and 1000 other words (articles, prepositions, pronouns, adjectives, adverbs, etc.) for a total of 10,000 words, which would be around 15 words per day for two years.
10 Answers
All I know is that the more words I memorize (I make a list of about 20 a week and create flashcards with sentences) the more I understand spoken Spanish.
Sometimes I read a word in the newspaper that I think "when will I ever need this word?," the word "brújula," for example. But then I heard the word on the radio during an interview and instantly knew what topic they were discussing.
A large vocabulary also opens up a whole new world in terms of how you express yourself. It's so much easier to pick the right words to say when you have a whole arsenal of words ready at your disposal.
Fluency sometimes seems like an impossible goal, yet it is one I dream about and work hard toward with the help of this website and the encouraging people on the answers forum.
We have had other discussions here on this topic. The conclusion we came to, if I remember correctly, is that if you take lessons with a native speaker, you study every day, consciously add to your vocabulary and have the opportunity for total emersion in a Spanish speaking country, you could be fluent in two years.
Your calculation comes to the same conclusion, but you are getting there by a different strategy.
For me, one down, one to go.
I have never thought of fluency in terms of number of words in one's vocabulary.
I recommend considering minimal fluency to be that point at which you can say everything you need to say in everyday conversation: at the grocery, in a restaurant, at the laundromat, asking directions, using polite phrases such a "con permiso", "no se apure", "no tengas cuidado", "lo siento" etc.
In other words, as we say in Spanish, "poder defenderse." 
Just an interesting aside, take a look at a Spanish-English dictionary and you'll notice the Spanish side is much thinner. Spansih does a great job of having many related words share their roots (verbs and nouns). Picking up Spanish vocabulary is a lot easier than picking up English vocabulary.
It is clear, words are the building blocks of any language. If you don't know words, it is impossible for you to be able to communicate.
But in my opinion, having a large vocabulary is not being able to recognize many words, but knowing how to use them. I often find myself saying the asme words over and over again, and although I know the meaning of many others, I can't find the right time to say them, or they just don't come to me at the appropriate moment, i.e. during a conversation. In English, I'm missing the glue that binds the bricks together.
I find this very frustrating and I feel helpless. Sometimes saying anything terrifies me because I can't find the words I want to use and I think it may sound bad, rude or wrong. At other times, I'm not sure if the word I'm thinking of exists or if I invented it. Conclusion: vocabulary is neccessary.
But I would say that in my usual Spanish usage this also happens on a smaller scale. I think the majority of Spanish speakers (I include myself) on a normal day don't use more than 1000 words, and 600 of them are related to work. We have a great tendency to use the same words and phrases; it is the law of least effort. And when someone uses an unusual word it sounds humorous. And, despite that, we all consider ourselves fluent. So it is not that difficult, Lorenzo!
The truth is that I have enriched my vocabulary in my own language primarily through reading.Iit was the only way to learn to write and express myself freely. If today I dare to write what I feel, I'm sure I owe it to books. So this practice is what I recommend.
Excellent advice, as always. 
For certain degree of fluency it is more important to be able to use (and not just passively understand) the most important words confidently, even if your overall vocabulary is very limited. For this you probably require at least 2000-3000. But you need to know about 5000-6000 dictionary words to be able to talk about most general subjects without having to be constantly looking for words you are missing.
Over 10,000 dictionary words you have a good vocabulary that includes many technical and specific words.
Over 20,000 dictionary words you are likely to know more words than many natives.
A dictionary of 30,000 words contains a lot of words that most natives do not know.
Over 40,000 dictionary words your vocabulary is unusually high. Over 50,000 and 60,000 you are a walking dictionary. Over 60,000 your vocabulary surpasses many thick single-volume dictionaries. Over 90,000 you've beaten the DRAE.
It is clear, words are the building blocks of any language. If you don't know words, it is impossible that you are able to communicate.
But in my opinion, to have a large vocabulary is not to be able to recognize many words, but to know how to use them. I often find myself saying once and again the same words, and although I know the meaning of many others, I can't find the right time to say them, or simply they don't come in the appropriate moment, when they should, i.e., during a conversation. In English, I'm missing the glue that binds the bricks.
And I find this very frustrating and I feel helpless. Sometimes even to say something terrifies me because I don't find the words I would want to use and I think it may sound badly, rude or wrong. Another times, I'm not sure if the word I'm thinking exists or I invented it. Conclusion, vocabulary is neccessary.
But I would say that in my common Spanish, on a smaller scale, this also happens. I think the majority of Spanish speakers ( I include myself) on a normal day don't spend nor 1000 words, and 600 of them are related to job. We have a great tendency to use words and recurring phrases, it is the law of least effort. Even when someone uses an unusual word, to others it sounds like humorous. And, despite that, we all consider ourselves fluents. So, it is not so difficult, Lorenzo!
The truth is that I have enriched my vocabulary in my own language primarily through reading. And it was the only way to learn to write and express myself with certain property. If today I dare to write what I feel, I'm sure I owe it to books. So, this practice is what I can recommend.
I like GablesLizzie's answer - besides, she has a really cool name, and I bet she's a hot lady, too 
I tend to think along the same terms: I started feeling comfortable with the term "fluent" when I realized I could understand effortlessly just about everything that was said on the radio or on TV, most of what I read, and of course, I could interact smoothly with English speakers for my daily activities, without them stopping every other sentence to correct (or laugh at) my English.
After that I started working on my accent, so that the "where are you from?" questions became a bit less frequent. 
2,000 sentences
A moderately large vocabulary (I don't know a number) is important for fluency, but it's only one piece. If you can't put the words together in a meaningful way, it doesn't matter how many you know.
One rule of thumb I like is that you should be able to read a newspaper without looking up a lot of words.
