diff --git a/src/config/index.ts b/src/config/index.ts
index 2eec012..b09ca54 100644
--- a/src/config/index.ts
+++ b/src/config/index.ts
@@ -64,20 +64,39 @@ export const API_ENDPOINTS = {
   },
 } as const;
 
-/**
- * Vault storage key names (AsyncStorage keys only — not user-editable "initial values").
- * - These are constants: the key names used to read/write/remove vault state in AsyncStorage.
- * - The actual stored values (S0 data, '1') are set by the app; do not change these key strings
- *   unless you are migrating storage (changing them would make existing data unfindable).
- * - Placed in config so VaultScreen and MeScreen (and others) use the same keys in one place.
- * - INITIALIZED: app sets to '1' after first mnemonic flow; SHARE_DEVICE: app stores serialized S0.
- * - "Reset Vault State" = remove both keys; next vault open sees no S0 and shows mnemonic flow.
- */
+// =============================================================================
+// Vault storage (user-isolated, multi-account)
+// =============================================================================
+// - AsyncStorage keys for vault state (S0 share, initialized flag).
+// - User-scoped: each account has its own keys so vault state is isolated.
+// - Store: use getVaultStorageKeys(userId) and write to INITIALIZED / SHARE_DEVICE.
+// - Clear: use same keys in multiRemove (e.g. MeScreen Reset Vault State).
+// - Multi-account: same device, multiple users → each has independent vault (no cross-user leakage).
+
+const VAULT_KEY_PREFIX = 'sentinel_vault';
+
+/** Base key names (for reference). Prefer getVaultStorageKeys(userId) for all reads/writes. */
 export const VAULT_STORAGE_KEYS = {
   INITIALIZED: 'sentinel_vault_initialized',
   SHARE_DEVICE: 'sentinel_vault_s0',
 } as const;
 
+/**
+ * Returns vault storage keys for the given user (user isolation).
+ * - Use for: reading S0, writing S0 after mnemonic, clearing on Reset Vault State.
+ * - userId null → guest namespace (_guest). userId set → per-user namespace (_u{userId}).
+ */
+export function getVaultStorageKeys(userId: number | string | null): {
+  INITIALIZED: string;
+  SHARE_DEVICE: string;
+} {
+  const suffix = userId != null ? `_u${userId}` : '_guest';
+  return {
+    INITIALIZED: `${VAULT_KEY_PREFIX}_initialized${suffix}`,
+    SHARE_DEVICE: `${VAULT_KEY_PREFIX}_s0${suffix}`,
+  };
+}
+
 // =============================================================================
 // Helper Functions
 // =============================================================================
diff --git a/src/screens/MeScreen.tsx b/src/screens/MeScreen.tsx
index 24e360b..7f97aba 100644
--- a/src/screens/MeScreen.tsx
+++ b/src/screens/MeScreen.tsx
@@ -18,7 +18,7 @@ import AsyncStorage from '@react-native-async-storage/async-storage';
 import { useAuth } from '../context/AuthContext';
 import { Heir, HeirStatus, PaymentStrategy } from '../types';
 import HeritageScreen from './HeritageScreen';
-import { VAULT_STORAGE_KEYS } from '../config';
+import { getVaultStorageKeys } from '../config';
 
 // Mock heirs data
 const initialHeirs: Heir[] = [
@@ -310,10 +310,11 @@ export default function MeScreen() {
 
   const handleResetVault = async () => {
     setResetVaultFeedback({ status: 'idle', message: '' });
+    const vaultKeys = getVaultStorageKeys(user?.id ?? null);
     try {
       await AsyncStorage.multiRemove([
-        VAULT_STORAGE_KEYS.INITIALIZED,
-        VAULT_STORAGE_KEYS.SHARE_DEVICE,
+        vaultKeys.INITIALIZED,
+        vaultKeys.SHARE_DEVICE,
       ]);
       setResetVaultFeedback({
         status: 'success',
diff --git a/src/screens/VaultScreen.tsx b/src/screens/VaultScreen.tsx
index d7d9576..749701e 100644
--- a/src/screens/VaultScreen.tsx
+++ b/src/screens/VaultScreen.tsx
@@ -25,7 +25,7 @@ import { VaultAsset, VaultAssetType, Heir } from '../types';
 import BiometricModal from '../components/common/BiometricModal';
 import { useAuth } from '../context/AuthContext';
 import { useVaultAssets } from '../hooks/useVaultAssets';
-import { VAULT_STORAGE_KEYS } from '../config';
+import { getVaultStorageKeys } from '../config';
 import { mnemonicToEntropy, splitSecret, serializeShare } from '../utils/sss';
 
 // Asset type configuration with nautical theme
@@ -183,15 +183,16 @@ export default function VaultScreen() {
   const [progressIndex, setProgressIndex] = useState(0);
   const [progressAnim] = useState(new Animated.Value(0));
   const { user, token } = useAuth();
+  const vaultKeys = React.useMemo(() => getVaultStorageKeys(user?.id ?? null), [user?.id]);
   const [isCapturing, setIsCapturing] = useState(false);
   const [treasureContent, setTreasureContent] = useState('');
   const mnemonicRef = useRef<View>(null);
   const progressTimerRef = useRef<ReturnType<typeof setInterval> | null>(null);
 
-  // Detect S0 (TEE/SE): if present, later open shows biometric only; if not, mnemonic flow
+  // Detect S0 (TEE/SE) for current user: if present, later open shows biometric only; if not, mnemonic flow
   useEffect(() => {
     let cancelled = false;
-    AsyncStorage.getItem(VAULT_STORAGE_KEYS.SHARE_DEVICE)
+    AsyncStorage.getItem(vaultKeys.SHARE_DEVICE)
       .then((v) => {
         if (!cancelled) setHasS0(!!v);
       })
@@ -199,7 +200,7 @@ export default function VaultScreen() {
         if (!cancelled) setHasS0(false);
       });
     return () => { cancelled = true; };
-  }, []);
+  }, [vaultKeys.SHARE_DEVICE]);
 
   // Only when S0 exists and vault not unlocked: show biometric after short delay.
   // When hasS0 is false or null, never show biometric — go straight to mnemonic flow.
@@ -379,9 +380,9 @@ export default function VaultScreen() {
       const entropy = mnemonicToEntropy(mnemonicWords, wordList);
       const shares = splitSecret(entropy);
       const s0 = shares[0]; // device share (S0)
-      // S0 is stored in AsyncStorage under SHARE_DEVICE — app-level storage, not hardware TEE/SE
-      await AsyncStorage.setItem(VAULT_STORAGE_KEYS.SHARE_DEVICE, serializeShare(s0));
-      await AsyncStorage.setItem(VAULT_STORAGE_KEYS.INITIALIZED, '1');
+      // S0 is stored in AsyncStorage under user-scoped key — app-level storage, not hardware TEE/SE
+      await AsyncStorage.setItem(vaultKeys.SHARE_DEVICE, serializeShare(s0));
+      await AsyncStorage.setItem(vaultKeys.INITIALIZED, '1');
       setHasS0(true);
       setShowMnemonic(false);
       setShowBiometric(true);