Fallout vegas: how do I gamble at vicki & vance? i waited 5 days already after getting sheriff?

You can't gamble there, the first place you can gamble at is near Vegas city, in a place slums sort of area just outside the city (to the south), follow the quests and it will lead you there in due time, enjoy the gameplay in the meantime. You can play 'caravan' with many of the traveling merchants (if you have completed the quests at the first town and gotten the deck of cards)