Do you have an advanced vocabulary?
To some extent. It's not extensive by any means, but it's far from simple.
I used to learn new and exotic words by nature in school, but now new words are few and far between, although I do plan on seriously upgrading my vocabulary soon.
Where did you gain your lexicon?
Most of my lexicon either came from reading books, Encyclopedias, online argument, and philosophy articles.
People often use advanced vocabulary online and while writing philosophical works/articles. In reading this material, I would often research what all of these words meant on online dictionaries. I did this all throughout high school. Ultimately, I gained a heap of insight into the subtle, nuanced definitions of various words, along with a plethora of new, intriguing and interesting words. Often times, I'll use words that seem to stun people, as if they are surprised to have heard a word used in that particular manner.
Additionally, while writing privately (which I used to do constantly as a teen) and responding to people on forums, where I often love to debate and discuss ideas, I usually have a great need to express my thoughts as clearly as possible. Personally, I have a preference for ornamentation, figures of speech, speaking in colors and abstractions, beautiful phrases that poetically roll off the tongue, rhetoric, and this need and desire to make everything absolutely crystal. Thus, I'll usually spend 30 minutes or more responding to one post at a time, making sure I've built my thoughts up sufficiently, point by point, so that every little facet of my thought is conveyed with vibrant brilliance and the most appropriate word I could manage to find to convey it. This resulted in my spending numerous hours on forums composing serious responses, whereupon I would research through dictionaries and thesauruses to find the most magnificent words to express my thoughts, as I often tend to highly value my ability to think with clarity and logical precision, in a very serious manner.
This, along with wanting to understand words which I did know or understand resulted in my larger vocabulary. If someone used a word I liked, I looked it up. If I was unsatisfied with a word I had used to express my thoughts, I looked it up and found a better one (and learned the specific use of the new word). If I read an article or essay and did not understand a word, I looked it up. So, I did a lot of research over the years, and this naturally resulted in my current lexicon (although I have somewhat lost the natural use of many of these words due to recent inactivity).
Are you fluent with them in speech or restricted to writing?
I am mostly restricted to writing. I have had many ENTP friends and often envied their ability to speak eloquently without much effort, as if their speech was what I hear in my head and write down on paper. Thus, in high school I actually spent some time trying to increase my oratory skills, yet to no avail. The more I tried, the less I was capable. It seems I'm just destined to write elegantly, yet doomed to speak in a babbling form, where my words often escape me. Every now and then, I'll manage to utilize a polysyllabic word in speech, but it's very rare and it requires much concentration and effort. So yeah, I'm great at writing, but I'm often found struggling to speak well.