There is no complete feature set for WhatsApp on PS Vita because the app does not exist for that platform. For any modern messaging experience, use a smartphone.
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Historically, some users tried accessing WhatsApp Web through the Vita's built-in browser. However, the Vita’s browser is outdated and lacks the modern web standards required to render the interface or scan the necessary QR codes.
For those who want a cleaner experience than the browser, some developers have created simple "app wrappers" that embed the WhatsApp Web interface into a standalone Vita app. These are usually found on homebrew repositories like VitaDB.