diff --git a/CHANGELOG.md b/CHANGELOG.md
index 16148eeae63..a81d8a57682 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -92,6 +92,7 @@
 * [BUGFIX] Parquet Converter: Fix `auto_forget_delay` having no effect. The ring lifecycler was created without the auto-forget delegate, so unhealthy instances were never automatically removed from the ring. #7752
 * [BUGFIX] Alertmanager: Reject the global `mattermost_webhook_url_file` setting in per-tenant configs, consistent with every other global `*_file` setting. #7768
 * [BUGFIX] Alertmanager: Tighten per-tenant config validation to reject additional file-based settings. #7767
+* [BUGFIX] Query Frontend: Disable vertical sharding for queries using `vector()`. `vector()` is not backed by any series selector, so every shard produced its sample and a query such as `<expr> or vector(0)` could return the `vector()` fallback instead of the left-hand side. #7806
 
 ## 1.21.1 2026-06-04
 
diff --git a/pkg/cortex/modules.go b/pkg/cortex/modules.go
index 1106a458e38..44f1855be1f 100644
--- a/pkg/cortex/modules.go
+++ b/pkg/cortex/modules.go
@@ -548,6 +548,10 @@ func (t *Cortex) initFlusher() (serv services.Service, err error) {
 func (t *Cortex) initQueryFrontendTripperware() (serv services.Service, err error) {
 	var queryAnalyzer querysharding.Analyzer
 	queryAnalyzer = querysharding.NewQueryAnalyzer()
+	// `vector()` synthesises a series that is not backed by any selector, so every
+	// vertical shard produces it. Sharding such a query can return the `vector()`
+	// fallback instead of the real data, so disable sharding for those queries.
+	queryAnalyzer = cortexquerysharding.NewDisableVectorFunctionAnalyzer(queryAnalyzer)
 	if t.Cfg.Querier.EnableParquetQueryable {
 		// Disable vertical sharding for binary expression with ignore for parquet queryable.
 		queryAnalyzer = cortexquerysharding.NewDisableBinaryExpressionAnalyzer(queryAnalyzer)
diff --git a/pkg/querysharding/util.go b/pkg/querysharding/util.go
index 05a8552cc32..da2b5816eb3 100644
--- a/pkg/querysharding/util.go
+++ b/pkg/querysharding/util.go
@@ -82,6 +82,61 @@ func ExtractShardingMatchers(matchers []*labels.Matcher) ([]*labels.Matcher, *st
 	return r, shardInfo.Matcher(&Buffers), nil
 }
 
+type disableVectorFunctionAnalyzer struct {
+	analyzer querysharding.Analyzer
+}
+
+// NewDisableVectorFunctionAnalyzer is a wrapper around the analyzer that marks queries
+// using the `vector()` function as not shardable.
+//
+// Vertical sharding is implemented by appending a shard matcher to every vector
+// selector, so each shard only evaluates the query over the series it owns. That is
+// only correct as long as every output series of the query is derived from a selector,
+// because then the shard that owns the series is the only shard that can produce it.
+//
+// `vector(s)` breaks that invariant: it synthesises a series with an empty labelset out
+// of a scalar, with no selector behind it, so *every* shard produces it. When that
+// sample can reach the query result - most notably as the right hand side of `or`, the
+// idiomatic "default value" pattern - the shards where the left hand side is empty emit
+// the `vector()` sample, and merging the shards can pick it over the real sample
+// produced by the shard that does own the data. The result is that a sharded query
+// returns the `vector()` fallback where an unsharded query correctly returns the left
+// hand side.
+//
+// This mirrors the way the upstream Thanos analyzer already refuses to shard `absent`,
+// `absent_over_time` and `scalar`, whose results likewise depend on data a single shard
+// cannot see. See https://github.com/cortexproject/cortex/issues/7804.
+func NewDisableVectorFunctionAnalyzer(analyzer querysharding.Analyzer) *disableVectorFunctionAnalyzer {
+	return &disableVectorFunctionAnalyzer{analyzer: analyzer}
+}
+
+func (d *disableVectorFunctionAnalyzer) Analyze(query string) (querysharding.QueryAnalysis, error) {
+	analysis, err := d.analyzer.Analyze(query)
+	if err != nil || !analysis.IsShardable() {
+		return analysis, err
+	}
+
+	expr, err := cortexparser.ParseExpr(query)
+	if err != nil {
+		// The wrapped analyzer already parsed the query successfully, so this should not
+		// happen. Be conservative and keep the wrapped analyzer's answer.
+		return analysis, nil
+	}
+	isShardable := true
+	parser.Inspect(expr, func(node parser.Node, nodes []parser.Node) error {
+		if n, ok := node.(*parser.Call); ok && n.Func != nil && n.Func.Name == "vector" {
+			isShardable = false
+			return stop
+		}
+		return nil
+	})
+	if !isShardable {
+		// Mark as not shardable.
+		return querysharding.QueryAnalysis{}, nil
+	}
+	return analysis, nil
+}
+
 type disableBinaryExpressionAnalyzer struct {
 	analyzer querysharding.Analyzer
 }
diff --git a/pkg/querysharding/util_test.go b/pkg/querysharding/util_test.go
index cba23190723..54235484c05 100644
--- a/pkg/querysharding/util_test.go
+++ b/pkg/querysharding/util_test.go
@@ -8,6 +8,75 @@ import (
 	"github.com/thanos-io/thanos/pkg/querysharding"
 )
 
+func TestDisableVectorFunctionAnalyzer_Analyze(t *testing.T) {
+	tests := []struct {
+		name            string
+		query           string
+		expectShardable bool
+		expectError     bool
+		description     string
+	}{
+		{
+			name:            "aggregation without vector()",
+			query:           `sum(rate(http_requests_total[5m])) by (job)`,
+			expectShardable: true,
+			expectError:     false,
+			description:     "Queries not using vector() are unaffected",
+		},
+		{
+			name:            "or vector() fallback",
+			query:           `sum(rate(http_requests_total[5m])) by (job) or vector(0)`,
+			expectShardable: false,
+			expectError:     false,
+			description:     "vector() is produced by every shard, so the query is not shardable",
+		},
+		{
+			name:            "vector() nested in an or operand",
+			query:           `max without (__name__, job, series) (test_series_a) or (-test_series_b or vector(-0.35))`,
+			expectShardable: false,
+			expectError:     false,
+			description:     "vector() anywhere in the query makes it not shardable",
+		},
+		{
+			name:            "vector() as an argument of another function",
+			query:           `sum(clamp_min(vector(1), 0)) by (job)`,
+			expectShardable: false,
+			expectError:     false,
+			description:     "vector() nested inside a call still makes the query not shardable",
+		},
+		{
+			name:            "series named vector",
+			query:           `sum(vector_series) by (job)`,
+			expectShardable: true,
+			expectError:     false,
+			description:     "A selector whose name merely contains vector must stay shardable",
+		},
+		{
+			name:            "invalid query",
+			query:           "invalid{query",
+			expectShardable: false,
+			expectError:     true,
+			description:     "Invalid queries should return error",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			analyzer := NewDisableVectorFunctionAnalyzer(querysharding.NewQueryAnalyzer())
+
+			result, err := analyzer.Analyze(tt.query)
+
+			if tt.expectError {
+				require.Error(t, err, tt.description)
+				return
+			}
+
+			require.NoError(t, err, tt.description)
+			assert.Equal(t, tt.expectShardable, result.IsShardable(), tt.description)
+		})
+	}
+}
+
 func TestDisableBinaryExpressionAnalyzer_Analyze(t *testing.T) {
 	tests := []struct {
 		name            string