This commit addresses below issues:

1. Short Format Subinterface getting deleted after vrf binding . Issue
10802:sonic-net/sonic-buildimage#10802
2. Do not allow short format subinterface to be created without encap
vlan configuration. Issue 10878:
sonic-net/sonic-buildimage#10878

config/main.py

@@ -6810,6 +6810,10 @@ def add_subinterface(ctx, subinterface_name, vid):
     subintf_dict = {}
     if vid is not None:
         subintf_dict.update({"vlan" : vid})
+
+    if subintf_vlan_check(config_db, get_intf_longname(interface_alias), vid) is True:
+        ctx.fail("Vlan {} encap already configured on other subinterface on {}".format(vid, interface_alias))
+
     subintf_dict.update({"admin_status" : "up"})
 
     try: