Typically Windows VPS's through Virtuozzo don't require a key so you don't have to put in your own. All that is required of the VPS provider is to tell microsoft of the number of CAL's being used so in your case i'm not sure - it's not like having a dedicated server where you can easily supply your own key.
If you are able to demonstrate you actually do have a valid license then all the VPS host would need to do is not report the CAL being used on your VPS - I know thats the way we would do it
however every host is different and some may well not bring down the cost based on this as the admin / billing overhead would be a pain!
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