coming into the US as of todays rate $10000 USD is equal to $13097 CAD so you would have to declare to CBP over that amount CAD, on the way back from Vegas you declare it at CBP in terminal 3. The process is simple, fill out a form and your on your way, unless they flag you for some reason, the real pain is leaving the US, you have to find the customs office and wait for an inspector to make the declaration. It could be right away or may take awhile if there are other people first (not only declarations but picking up delayed luggage etc.).