In English, Singular Subject Complement [ Predicate Noun / Nominative ] uses the [ Null / Zero Determiner / Article ].
The Subject Complement Plural [ Predicate Noun / Nominative ], Singular and Plural [ Predicate Adjective ] forms do not use the [ Null / Zero / Determiner Article ].
Many words may be used as both a Noun or an Adjective. Some of these dual grammatical function words, when used in a Subject Complement Clause or Sentence, may with the selection of a Null / Zero Determiner / Article, Definite Article, Indefinite Article or other Determiner, transform the otherwise same word phrase to being a either a [ Predicate Noun / Nominative ] or a [ Predicate Adjective ] Subject Complement.
Some Subject Complement phrases with the exact same words may be either Predicate Adjective or Nominative, depending on the intended or contextual meaning:
It is juvenile [ Pred Adj ]
It is a juvenile [ Pred Nom ]
It is mineral, vegetable or animal [ Pred Adj ]
It is a mineral, a vegetable or an animal [ Pred Nom ]
It is German [ Pred Adj - it is of Germanic origin, influence ]
The exact same words being Predicate Adjective or Nominative:
That is some German [ Pred Adj - something is depicted as having some Germanic attributes ]
That is some German [ Pred Nom - some person is considered to be German and a German • as a stranger, acquaintance, guest, nondescript, notable, etc. ]
Singular Predicate Noun / Nominative - not Null / Zero Determiner / Article:
I am a fly.
You are a fly.
She / He / It is a fly.
Plural Predicate Noun / Nominative - Null / Zero Determiner / Article:
We are flies.
You all are flies.
They are flies.
I'm Asian [ Pred Adj ]
I'm an Asian [ Pred Nom ]
I'm artistic [ Pred Adj ]
I'm an artist [ Pred Nom ]
We're Asian [ Pred Adj ]
We're Asians [ Pred Nom ]
We're artistic [ Pred Adj ]
We're artists [ Pred Nom ]
The Es in this word (and often times in Portuguese) are close to the English E sound in the word in "let" and not like either one you listed. The O at the end sounds closed to the 'oo' sound at the end of "kangaroo" in Brazil. If you type "vegetariano" into google translate Portuguese, it will sound it out for you very accurate to my ear. Their pronounciation isn't too far off here.
"Vegetarian" can both be a noun and an adjective. "I am vegetarian" (adjective) and "I am a vegetarian" (noun), although they mean the same thing, are grammatically different. Does the same difference existence in Portuguese or should I never use the indefinite article if I'm calling myself a vegetarian?
(I'm not a vegetarian so it's not something I'll ever say, but it's nice to know)