Questions & Answers about El vacío parece profundo.
Here vacío is being used as a noun, not as an adjective.
- vacío as an adjective means empty
- un vaso vacío = an empty glass
- el vacío as a noun means something like the void, emptiness, or empty space
The article el helps show that it is a noun here.
Also, in Spanish, abstract nouns often appear with the definite article:
- el amor = love
- la vida = life
- el vacío = the void / emptiness
So El vacío parece profundo is a natural way to say it.