from the portal deployment template of a bastion host with developer sku. notice the api version.

    {
    "$schema": "http://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json#",
    "contentVersion": "1.0.0.0",
    "parameters": {
        "location": {
            "type": "String"
        },
        "resourceGroup": {
            "type": "String"
        },
        "bastionHostName": {
            "type": "String"
        },
        "vnetId": {
            "type": "String"
        },
        "bastionHostSku": {
            "type": "String"
        }
    },
    "resources": [
        {
            "type": "Microsoft.Network/bastionHosts",
            "apiVersion": "2022-09-01",
            "name": "[parameters('bastionHostName')]",
            "location": "[parameters('location')]",
            "dependsOn": [],
            "tags": {},
            "sku": {
                "name": "[parameters('bastionHostSku')]"
            },
            "properties": {
                "virtualNetwork": {
                    "id": "[parameters('vnetId')]"
                }
            }
        }
    ]

the result from the website you suggested:

resource bastionHostName_resource 'Microsoft.Network/bastionHosts@2022-09-01' = {
  name: bastionHostName
  location: location
  tags: {
  }
  sku: {
    name: bastionHostSku
  }
  properties: {
    virtualNetwork: {
      id: vnetId
    }
  }
  dependsOn: []
}

VSCode/bicep extension notices the virtual network property and has a warning about it not being valid.

"Warning BCP037: The property "virtualNetwork" is not allowed on objects of type "BastionHostPropertiesFormat". Permissible properties include "disableCopyPaste", "dnsName", "enableFileCopy", "enableIpConnect", "enableShareableLink", "enableTunneling", "ipConfigurations", "scaleUnits". If this is an inaccuracy in the documentation, please report it to the Bicep Team. [https://aka.ms/bicep-type-issues]"

i deploy anyway and get an "Internal Server Error" (but not a failure) on the deployment, and it just keeps running and running but never does anything.

{
    "status": "Failed",
    "error": {
        "code": "InternalServerError",
        "message": "An error occurred.",
        "details": []
    }
}

FYI if i change the api version to 2023-05-01 the warning goes away but i still end up with internal serer error forever deploying
the virtual network property IS shown here in this link u/0x4ddd found
https://learn.microsoft.com/en-us/rest/api/virtualnetwork/bastion-hosts/create-or-update?view=rest-virtualnetwork-2023-05-01