Scamble a vector, preserving the sum
I am also relatively new to Clojure, but here are my first thoughts:


I like that you work with a vector, because random access of vectors with (nth) and assoc is faster. I also like your naming. thanks for providing a deftest, but please, user a proper inlining.
(assoc)'s do not need to be nested: (assoc (assoc c n+ v+) n- v-) is the same as (assoc c n+ v+ n- v-).
(last (take n x)) can be written as (nth x n).
Warning: you use the name 'first in yout (let), but this name is already defined in clojure.core.
For a style guide, please see https://github.com/bbatsov/clojure-style-guide


That is all for now, and I hope you found some advice useful.