Custom virtual network is required

Custom virtual network is required, so calling it out explicitly

Author: ChiragPavecha

articles/aks/virtual-nodes-cli.md

@@ -58,6 +58,9 @@ az group create --name myResourceGroup --location westus
 
 ## Create a virtual network
 
+> [!IMPORTANT]
+> Virtual Node requires to have a custom virtual network and associated subnet. It can't be associated with the AKS cluster virtual network.
+
 Create a virtual network using the [az network vnet create][az-network-vnet-create] command. The following example creates a virtual network name *myVnet* with an address prefix of *10.0.0.0/8*, and a subnet named *myAKSSubnet*. The address prefix of this subnet defaults to *10.240.0.0/16*:
 
 ```azurecli-interactive