For an ordinary singular proper-name vocative use yā Zaydu, not yā Zaydan. An iḍāfa vocative such as yā ʿAbda llāhi is visibly accusative.
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A Common Mistake: Yā Zaydan
Vocative Particles: Five Forms, Uses and Parsing
For an ordinary singular proper-name vocative use yā Zaydu, not yā Zaydan. An iḍāfa vocative such as yā ʿAbda llāhi is visibly accusative.