I don't think that you'll be able to get a suitable direct flight from Manchester to LasVegas.
The Manchester Airport website lists only BMI (scheduled) and MyTravel (charter) flights on this route. The MyTravel flights are only weekly, so they're no use for a 5 night stay. The BMI website states that they fly direct to Las Vegas on Tuesdays (summer schedule only), Thursdays and Sundays but this information seems to be out of date. Searching for flights on these days only brings up the connecting service, via Chicago.
If it has to be Manchester that you fly from, the American Airlines flights, connecting at Chicago, offered by eBookers do seem to be the best offers available. (I've checked American Airlines own site and can't find anything cheaper). eBookers are currently offering a fare of �467, departing on the 4th and returning on the 9th. Travel around the Easter weekend period is usually expensive on most routes. Some full-fare airlines are currently quoting flexible economy fares of around �3000 for Manchester to Las Vegas (via Chicago) around that time, so anything under �1000 should be regarded as a bargain. A fare of less than �500 should definitely be grabbed as soon as possible!
By the way, I've spent quite a bit of time researching whether its possible to fly, from Manchester, to a different US airport and then transfer to a budget carrier within the USA. I still can't find any cheaper fares.
I've also checked US aIrways deals. The cheapest I can find is �484.80 (through Opodo. Out 4th, back 9th, 1change of aircraft).
Chris