Open a ticket at http://support.myvegas.com/requests/new I've canceled two rewards and both were processed in under 24 hours
Call the number in the email, it is the MLife desk at the hotel where the offer is good at. I have done it twice and get credit by the next day.
That is hit and miss. I had a reward that they could not refund; I spoke at length with a desk manager and they just couldn't do it. I had to open a ticket which is easy enough.
I used that web link a few days ago to request a refund. The refund was done the next day. Easiest way to do it.
I have opened a ticket twice for a refund. The first was done within a matter of minutes, the second was the next day. Easy process.