AND, I want to back up further here and ask: is there research that demonstrates beyond a shadow of a doubt that it is vocabulary that defines whether a text is complex or not? If there is, I would love to see the research. Even CCSS examines factors beyond vocabulary.
If we judge complexity solely on vocabulary using Lexile and reading level measures, here is what happens: books end up being placed in the wrong hands. Two quick examples:
CIRCUS MIRANDUS is a debut novel by Cassie Beasley. The Lexile for this book places it squarely in the middle of the band for grades 2-3. BONE GAP by Laura Ruby has a Lexile that places it in the 4-5 grade band. If you have not read either book, this guideline of a Lexile might cause you to think there are relatively simple if not simplistic books. Nothing could be further from the truth. Here is a link to one of the starred reviews for Circus Mirandus (which is recommended for ages 9-12, BTW): http://www.publishersweekly.com/978-0-525-42843-5.
Bone Gap, recommended for 13-18 year olds, is reviewed here: https://www.kirkusreviews.com/book-reviews/laura-ruby/bone-gap/. It is one of MANY starred reviews.
I think both of these books qualify as complex reads. I think the critics agree. However, if we depend on vocabulary, these books miss the intended audience by a mile.
If you encounter these claims about vocabulary, about "research," about audience, please take time to question them, investigate them further. If your Spidey Sense tingles, chances are, there is something not quite right.