LibsDisguises & PlayerParticles

A disguise or a particle trail can give a vanished player away.

LibsDisguises

With LibsDisguises and hooks.libsdisguises: true:

• A disguised player who vanishes is undisguised.
• A vanished player can't put a disguise on.

Both send disguise-removed-because-vanish.

PlayerParticles

With PlayerParticles and hooks.playerparticles: true, a vanished player's active particles are cleared when they vanish and again every second while they stay vanished, in case they turn some back on.

This only clears the particles that are running. Their saved particle setup in PlayerParticles is not changed.

The action bar

Not a plugin, but in the same spirit: with hooks.actionbar: true (the default) vanished players see messages.action-bar (You are in vanish.) above their hotbar, refreshed every 1.5 seconds, so they never forget they're hidden. Set the message to "" to hide it.