diff --git a/audit.py b/audit.py
index cb34196..c12f48a 100755
--- a/audit.py
+++ b/audit.py
@@ -36,7 +36,7 @@
 from cli_audit.logging_config import setup_logging  # noqa: E402
 # Split file support (Phase 2.1)
 from cli_audit.upstream_cache import (  # noqa: E402
-    UpstreamVersion, UpstreamCache,
+    UpstreamVersion,
     load_upstream_cache, write_upstream_cache, get_upstream_cache_path,
     update_cached_upstream,
 )
@@ -1025,9 +1025,12 @@ def cmd_update_baseline(args: argparse.Namespace) -> int:
 
     print(f"# Collecting upstream versions for {total} tools...", file=sys.stderr)
 
-    # Collect upstream versions only
-    upstream_cache = UpstreamCache()
+    # Collect upstream versions only. Merge into the existing baseline so a
+    # tool-scoped run (or a transient collection failure) never drops the
+    # entries that were not collected this run (issue #125).
+    upstream_cache = load_upstream_cache()
     completed = 0
+    collected = 0
 
     with ThreadPoolExecutor(max_workers=min(MAX_WORKERS, total)) as executor:
         future_to_tool = {}
@@ -1048,6 +1051,7 @@ def cmd_update_baseline(args: argparse.Namespace) -> int:
                 )
                 upstream_cache.versions[tool.name] = version
                 completed += 1
+                collected += 1
 
                 display = latest_num or latest_tag or "n/a"
                 print(f"# [{completed}/{total}] {tool.name}: {display}", file=sys.stderr)
@@ -1059,7 +1063,7 @@ def cmd_update_baseline(args: argparse.Namespace) -> int:
     write_upstream_cache(upstream_cache)
     print("", file=sys.stderr)
     print(f"✓ Upstream baseline updated: {get_upstream_cache_path()}", file=sys.stderr)
-    print(f"✓ Collected {len(upstream_cache.versions)} versions", file=sys.stderr)
+    print(f"✓ Collected {collected}/{total} versions", file=sys.stderr)
 
     # Report rate limit
     rate_limit = get_github_rate_limit()
diff --git a/tests/test_update_fixes.py b/tests/test_update_fixes.py
index 8e0e294..31a27d3 100644
--- a/tests/test_update_fixes.py
+++ b/tests/test_update_fixes.py
@@ -1260,3 +1260,57 @@ def test_refreshes_installed_but_preserves_latest(self, tmp_path):
         assert entry["installed_version"] not in ("", "0.0.1"), "installed must be refreshed"
         # directional status: real git (e.g. 2.x) < 999.0.0 -> OUTDATED
         assert entry["status"] == "OUTDATED"
+
+
+@skip_on_windows
+class TestUpdateBaselineMerges:
+    """A tool-scoped `--update-baseline <tool>` must MERGE the collected entry
+    into the existing committed baseline, never replace the whole file with only
+    the requested tool (issue #125: `--update-baseline vault` dropped every other
+    entry). Failed collections must likewise leave the existing entry intact."""
+
+    def _seed_baseline(self, path):
+        path.write_text(json.dumps({
+            "__meta__": {"baseline_updated_at": "2026-01-01T00:00:00Z",
+                         "schema_version": 2, "source": "test"},
+            "versions": {
+                "keepme": {"latest_tag": "v1.0.0", "latest_version": "1.0.0",
+                           "latest_url": "", "tool_url": "", "upstream_method": "gh"},
+                "ripgrep": {"latest_tag": "v0.0.1", "latest_version": "0.0.1",
+                            "latest_url": "", "tool_url": "", "upstream_method": "gh"},
+            },
+        }))
+
+    def test_single_tool_update_preserves_other_entries(self, tmp_path, monkeypatch):
+        import argparse
+        import audit
+        baseline = tmp_path / "upstream_versions.json"
+        self._seed_baseline(baseline)
+        monkeypatch.setenv("CLI_AUDIT_UPSTREAM_FILE", str(baseline))
+
+        with patch.object(audit, "collect_latest_version", return_value=("v9.9.9", "9.9.9")), \
+                patch.object(audit, "get_github_rate_limit", return_value=None):
+            rc = audit.cmd_update_baseline(argparse.Namespace(tools=["ripgrep"]))
+
+        assert rc == 0
+        data = json.loads(baseline.read_text())
+        assert data["versions"]["ripgrep"]["latest_version"] == "9.9.9"
+        assert "keepme" in data["versions"], "unrelated entries must survive a tool-scoped run"
+        assert data["versions"]["keepme"]["latest_version"] == "1.0.0"
+
+    def test_failed_collection_keeps_existing_entry(self, tmp_path, monkeypatch):
+        import argparse
+        import audit
+        baseline = tmp_path / "upstream_versions.json"
+        self._seed_baseline(baseline)
+        monkeypatch.setenv("CLI_AUDIT_UPSTREAM_FILE", str(baseline))
+
+        with patch.object(audit, "collect_latest_version", side_effect=RuntimeError("network down")), \
+                patch.object(audit, "get_github_rate_limit", return_value=None):
+            rc = audit.cmd_update_baseline(argparse.Namespace(tools=["ripgrep"]))
+
+        assert rc == 0
+        data = json.loads(baseline.read_text())
+        # transient failure must not delete the committed entry
+        assert data["versions"]["ripgrep"]["latest_version"] == "0.0.1"
+        assert "keepme" in data["versions"]