`keyvault 2021-10-01`: `ignoreMissingVnetServiceEndpoint` not respond as expect

I created a keyvault with `ignoreMissingVnetServiceEndpoint = true` but the server does not respond `ignoreMissingVnetServiceEndpoint` field as `true` when i retrieve the keyvault.

request as:

```
curl -H 'content-type: application/json; charset=utf-8' -H 'user-agent: Go/go1.19.3 (amd64-linux) go-autorest/v14.2.1 Azure-SDK-For-Go/v66.0.0 keyvault/2021-10-01 HashiCorp Terraform/1.3.6 (+https://www.terraform.io) Terraform Plugin SDK/2.10.1 terraform-provider-azurerm/dev pid-222c6c49-1b0a-5959-a213-6608f9eb8820' -H 'authorization: Bearer xxxxx' -H 'x-ms-authorization-auxiliary: ' -H 'x-ms-correlation-request-id: 10f68284-3bfb-5132-9756-b3bf890b7aaf' --compressed
-X PATCH
'https://management.azure.com/subscriptions/xx-xx-x-x-xxx/resourceGroups/acctestRG-230110183813226776/providers/Microsoft.KeyVault/vaults/vault230110183813226776?api-version=2021-10-01'
-d '{
"properties": {
"networkAcls": {
"bypass": "AzureServices",
"defaultAction": "Allow",
"ipRules": [
{
"value": "123.0.0.101"
},
{
"value": "123.0.0.102/32"
}
],
"virtualNetworkRules": [
{
"id": "/subscriptions/xx-xx-x-x-xxx/resourceGroups/acctestrg-230110183813226776/providers/Microsoft.Network/virtualNetworks/acctestvirtnet230110183813226776/subnets/acctestsubneta230110183813226776",
"ignoreMissingVnetServiceEndpoint": false
},
{
"id": "/subscriptions/85b3dbca-5974-4067-9669-67a141095a76/resourceGroups/acctestrg-230110183813226776/providers/Microsoft.Network/virtualNetworks/acctestvirtnet230110183813226776/subnets/acctestsubnetb230110183813226776",
"ignoreMissingVnetServiceEndpoint": true
}
]
}
}
}'
```

and it response with no `ignoreMissingVnetServiceEndpoint`

```
HTTP/2.0 200
cache-control: no-cache
pragma: no-cache
content-type: application/json; charset=utf-8
expires: -1
vary: Accept-Encoding
x-ms-client-request-id: c8d75352-7a55-4e17-8f41-f2ace95b4f11
x-ms-keyvault-service-version: 1.5.606.0
x-ms-request-id: 53bf4605-d867-418d-a57d-1e79c1e70dad
x-content-type-options: nosniff
strict-transport-security: max-age=31536000; includeSubDomains
x-ms-ratelimit-remaining-subscription-writes: 1198
server: Microsoft-IIS/10.0
x-aspnet-version: 4.0.30319
x-ms-correlation-request-id: 10f68284-3bfb-5132-9756-b3bf890b7aaf
x-ms-routing-request-id: EASTASIA:20230110T104421Z:a14bbbf7-92d9-4f9b-8de6-f3428ab3f9af
date: Tue, 10 Jan 2023 10:44:21 GMT
content-length: 1755

{
"id": "/subscriptions/xxx-xx-xx-x-xx/resourceGroups/acctestRG-230110183813226776/providers/Microsoft.KeyVault/vaults/vault230110183813226776",
"name": "vault230110183813226776",
"type": "Microsoft.KeyVault/vaults",
"location": "westeurope",
"tags": {},
"properties": {
"sku": {
"family": "A",
"name": "standard"
},
"tenantId": "72f988bf-86f1-41af-91ab-2d7cd011db47",
"networkAcls": {
"bypass": "AzureServices",
"defaultAction": "Allow",
"ipRules": [
{
"value": "123.0.0.101/32"
},
{
"value": "123.0.0.102/32"
}
],
"virtualNetworkRules": [
{
"id": "/subscriptions/xxxx/resourcegroups/acctestrg-230110183813226776/providers/microsoft.network/virtualnetworks/acctestvirtnet230110183813226776/subnets/acctestsubneta230110183813226776"
# <-- no ignoreMissingVnetServiceEndpoint
},
{
"id": "/subscriptions/xxxx/resourcegroups/acctestrg-230110183813226776/providers/microsoft.network/virtualnetworks/acctestvirtnet230110183813226776/subnets/acctestsubnetb230110183813226776"
# <-- no ignoreMissingVnetServiceEndpoint
}
]
},
"accessPolicies": [
{
"tenantId": "72f988bf-86f1-41af-91ab-2d7cd011db47",
"objectId": "46f17840-0e65-493b-b5b7-caf7e2520c8c",
"permissions": {
"keys": [
"Create"
],
"secrets": [
"Set"
],
"certificates": [],
"storage": []
}
}
],
"enabledForDeployment": false,
"enabledForDiskEncryption": false,
"enabledForTemplateDeployment": false,
"enableSoftDelete": true,
"softDeleteRetentionInDays": 7,
"enableRbacAuthorization": false,
"vaultUri": "https://vault230110183813226776.vault.azure.net/",
"provisioningState": "Succeeded",
"publicNetworkAccess": "Enabled"
}
}
```

related API:
https://github.com/Azure/azure-rest-api-specs/blob/4b7373a29b0cc90e654ca66a8ca3742affca732d/specification/keyvault/resource-manager/Microsoft.KeyVault/stable/2021-10-01/keyvault.json#L1798-L1801

Start with the stable 2021-10-01 Key Vault schema at specification/keyvault/resource-manager/Microsoft.KeyVault/stable/2021-10-01/keyvault.json, especially lines 1798-1801, and compare it with the captured PATCH response. Done means establishing whether the API contract accurately describes the missing ignoreMissingVnetServiceEndpoint response field and recording the agreed behavior.

Written by the indexing model from the issue text.