you dont need to have Multiple key vaults and Mutiple keys for enabling OS and Data Disk Encryption. The scenario when you would need Multiple Key Vault if the Virtual machines are in a different region
For example if the VM is in East US region the KV ( key vault ) should also be in the same region as long as that critreria is met you can use the Keys and keyvault
The same key can be used but if needed be you should create multiple keys and lower the footprint of the attack vector