1. Ego, is nominative because it is the subject.
2. Vocatus is nominative because it is the predicate agreeing with the subject.
3. Johannes, is nominative because it is part of the predicate, and in apposition with vocatus.
N.B. Although in precise language Johannes is said to agree with vocatus rather than to be in apposition with it, the expression, as it stands, is correct. Apposition is the agreement of substantives, agreement the apposition of adjectives.
Rule 3.—All verbs which, when resolved into a copula and participle, have their participle in apposition (or agreeing) with the noun, are in the same condition as simple copulas—she walks a queen=she is walking a queen=illa est incedens regina.
Rule 4.—The construction of a subject and copula preceded by the conjunction that, is the same in respect to the predicate by which they are followed as if the sentence were an isolated proposition.
This rule determines the propriety of the expression—I believe that it is he as opposed to the expression I believe that it is him.
I believe=I am believing, and forms one proposition.
It is he, forms a second.
That, connects the two; but belongs to neither.