diff --git a/changelogs/fragments/42322-nmcli_fix_vlan_mod_function.yaml b/changelogs/fragments/42322-nmcli_fix_vlan_mod_function.yaml
new file mode 100644
index 0000000000..8abd773ae9
--- /dev/null
+++ b/changelogs/fragments/42322-nmcli_fix_vlan_mod_function.yaml
@@ -0,0 +1,2 @@
+bugfixes:
+  - "nmcli - fix syntax of vlan modification command (https://github.com/ansible/ansible/issues/42322)"
diff --git a/lib/ansible/modules/net_tools/nmcli.py b/lib/ansible/modules/net_tools/nmcli.py
index 90a989b050..1c0ec02ffb 100644
--- a/lib/ansible/modules/net_tools/nmcli.py
+++ b/lib/ansible/modules/net_tools/nmcli.py
@@ -1033,10 +1033,10 @@ class Nmcli(object):
 
         params = {'dev': self.vlandev,
                   'id': self.vlanid,
-                  'ip4': self.ip4,
-                  'gw4': self.gw4,
-                  'ip6': self.ip6,
-                  'gw6': self.gw6,
+                  'ip4': self.ip4 or '',
+                  'gw4': self.gw4 or '',
+                  'ip6': self.ip6 or '',
+                  'gw6': self.gw6 or '',
                   'autoconnect': self.bool_to_string(self.autoconnect)
                   }
         for k, v in params.items():
@@ -1048,16 +1048,22 @@ class Nmcli(object):
         cmd = [self.nmcli_bin]
         cmd.append('con')
         cmd.append('mod')
-        cmd.append('con-name')
+
+        if self.conn_name is not None:
+            cmd.append(self.conn_name)
+        elif self.ifname is not None:
+            cmd.append(self.ifname)
+        else:
+            cmd.append('vlan%s' % self.vlanid)
 
         params = {'vlan.parent': self.vlandev,
                   'vlan.id': self.vlanid,
-                  'ipv4.address': self.ip4,
-                  'ipv4.gateway': self.gw4,
-                  'ipv4.dns': self.dns4,
-                  'ipv6.address': self.ip6,
-                  'ipv6.gateway': self.gw6,
-                  'ipv6.dns': self.dns6,
+                  'ipv4.address': self.ip4 or '',
+                  'ipv4.gateway': self.gw4 or '',
+                  'ipv4.dns': self.dns4 or '',
+                  'ipv6.address': self.ip6 or '',
+                  'ipv6.gateway': self.gw6 or '',
+                  'ipv6.dns': self.dns6 or '',
                   'autoconnect': self.bool_to_string(self.autoconnect)
                   }
 
diff --git a/test/units/modules/net_tools/test_nmcli.py b/test/units/modules/net_tools/test_nmcli.py
index 42492d88a7..86713226d1 100644
--- a/test/units/modules/net_tools/test_nmcli.py
+++ b/test/units/modules/net_tools/test_nmcli.py
@@ -402,7 +402,15 @@ def test_create_vlan_con(mocked_generic_connection_create):
     arg_list = nmcli.Nmcli.execute_command.call_args_list
     args, kwargs = arg_list[0]
 
-    for param in ['vlan']:
+    assert args[0][0] == '/usr/bin/nmcli'
+    assert args[0][1] == 'con'
+    assert args[0][2] == 'add'
+    assert args[0][3] == 'type'
+    assert args[0][4] == 'vlan'
+    assert args[0][5] == 'con-name'
+    assert args[0][6] == 'non_existent_nw_device'
+
+    for param in ['ip4', '10.10.10.10', 'gw4', '10.10.10.1']:
         assert param in args[0]
 
 
@@ -419,7 +427,12 @@ def test_mod_vlan_conn(mocked_generic_connection_modify):
     arg_list = nmcli.Nmcli.execute_command.call_args_list
     args, kwargs = arg_list[0]
 
-    for param in ['vlan.id']:
+    assert args[0][0] == '/usr/bin/nmcli'
+    assert args[0][1] == 'con'
+    assert args[0][2] == 'mod'
+    assert args[0][3] == 'non_existent_nw_device'
+
+    for param in ['ipv4.address', '10.10.10.10', 'ipv4.gateway', '10.10.10.1']:
         assert param in args[0]