Don't worry about using simple language as long as it's well suited to your purposes. A good writer will make use of a large vocabulary because they recognize that synonyms don't have exactly the same meaning, and will try to use the word that most precisely represents the idea they want to get across. A poor writer may see the good writing, notice the large vocabulary, and seek to emulate it...by grabbing a thesaurus and using it to replace the words they originally used with sophisticated-sounding synonyms. That may make the writing look sophisticated, but to anyone who's familiar with what the words actually mean, it'll just come off as pretentious.
A particularly common case of this is the word "said". Quite a few poor writers hate having characters "say" something. They may articulate, vocalize, intone, report, recite, announce, voice, bark, or exclaim something, but certainly never just say it. Except usually when we say something, we do just "say" it, without bothering to do so in a special way. Good authors know this, and their writing will use the simple, but correct, "said".
Some very nice examples of this sort of pretentious and imprecise language are listed in <PERSON> "Fenimore Cooper's Literary Offenses". (He also talks about the same issues I brought up in the first paragraph, and does so far more eloquently than I did, while using relatively simple language throughout.) Note that while there are a couple of places where <PERSON>'s suggested replacement is more "sophisticated" (or at least less common) than the word being replaced, in most cases, the more common word is also the more correct one (which makes sense; words are common because the ideas they represent come up a lot).
<PERSON>'s word-sense was singularly dull. 873
When a person has a poor ear for music he will flat and sharp right along without knowing it. He keeps near the tune, but is not the tune. When a person has a poor ear for words, the result is a literary flatting and sharping; you perceive what he is intending to say, but you also perceive that he does not say it. This is <PERSON>. He was not a word-musician. His ear was satisfied with the approximate words. I will furnish some circumstantial evidence in support of this charge. My instances are gathered from half a dozen pages of the tale called "Deerslayer." He uses "Verbal" for "oral"; "precision" for "facility"; "phenomena" for "marvels"; "necessary" for "predetermined"; "unsophisticated" for "primitive"; "preparation" for "expectancy"; "rebuked" for "subdued"; "dependent on" for "resulting from"; "fact" for "condition"; "fact" for "conjecture"; "precaution" for "caution"; "explain" for "determine"; "mortified" for "disappointed"; "meretricious" for "factitious"; "materially" for "considerably"; "decreasing" for "deepening"; "increasing" for "disappearing"; "embedded" for "inclosed"; "treacherous" for "hostile"; "stood" for "stooped"; "softened" for "replaced"; "rejoined" for "remarked"; "situation" for "condition"; "different" for "differing"; "insensible" for "unsentient"; "brevity" for "celerity"; "distrusted" for "suspicious"; "mental imbecility" for "imbecility"; "eyes" for "sight"; "counteracting" for "opposing"; "funeral obsequies" for "obsequies."
When writing, focus on the idea you want to get across, and choose words that most clearly and precisely accomplish this task. The resulting text may or may not be fancy, but it will be elegant. 312