This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 蒕 is uncertain. It visually contains 艹 (grass) over 人 (person), 口 (mouth), and 皿 (dish), but the original semantic and phonetic roles of these components are unclear.
With no clear meaning, picture a person (人) with a mouth (口) under the grass (艹) holding a dish (皿) — an obscure scene that helps remember the shape.
For ウン, imagine a person saying 'un' (うん) while nodding under the grass with a dish: the sound うん cues ウン.
—