Learn Russian: Вдали я смутно видел какую-то фигуру. - In the distance, I vaguely saw some figure.

You could add it right after фигуру to express your doubt. For example: Вдали я смутно видел какую-то фигуру, то ли человека, то ли медведя (In the distance I vaguely saw some figure, whether a person or a bear). Because фигуру is in the accusative case (the direct object), the nouns you list with то ли... то ли... must also be in the accusative case.

Why is какую-то used here instead of какую-нибудь?

The suffix -то is used because the figure actually exists in reality—the speaker saw a real, specific object, they just can't identify exactly what it is. The suffix -нибудь is used for hypothetical or non-specific objects (meaning "any kind of"), which usually appear in questions, future tense, or commands (e.g., "Did you see any figure?").

Why is вдали written as a single word here?

Written as one word, вдали is an adverb meaning "in the distance" (answering the static question where?). If it were written as two words (в даль), it would be a preposition plus a noun meaning "into the distance." That two-word form implies movement or direction (answering the question where to?), such as in the phrase смотреть в даль (to look into the distance).

Could I use плохо instead of смутно to mean I saw it "poorly"?