Corrected entry: The Tropicana's "future stealing" scam seems to depend on Lorne's abilities too much to be as elaborate as depicted. The operation and black market in futures would have to have been in place well before he got there, so how were they doing it before Lorne showed up? Randomly taking destinies from people in the casino and just HOPING they were worthwhile? If so, what would they do with the people whose destinies they'd stolen? Let them play the slots until they die and then somehow dispose of the bodies or turn them loose in the streets of Vegas and draw the mortal world's attention to themselves?
Corrected entry: When Angel, Fred, and Gunn are trying to find where Lorne's performing, they come across his headliner ad at the Tropicana, which states "17th Straight Week." At the end of the last season Lorne was just leaving for Vegas, and the previous episodes have established that Angel and Cordelia have been missing for three months. Assuming Lorne started performing the week he arrived in Vegas, his show should be in its 14th straight week.
Correction: Firstly, three months is probably an approximation - it could be a week or two more than that. Secondly, some time must have passed between Angel's rescue and the trip to Las Vegas - he's had time to fully recover from his ordeal, look for Cordelia (there's the events of the episode "Ground State", for example) and so on. This could easily take at least a couple of weeks, which makes the 17 week notice quite possible.
Correction: Just because we don't know the way the casinos plan worked before Lorne turned up doesn't mean it couldn't have worked. For all we know they could have been using another demon or any number of techniques. A little imagination is all that's called for.