having been to all three I'd say you really can't go wrong in any way.
Depends on how ambitious or laid back you want to be.
Laos still has a lot of it's colonial heritage and is a great place to relax.
Vietnam is a study in contrasts as it modernizes in some places and stays nice and relaxed in others. It's also got the best beaches of the three.
Cambodia is by comparison rural compared to the two above places, but has the single biggest attraction in the three countries and you'd be hard pressed to say it was the least developed if all you really saw of the country was Siem Riep.
In terms of people - in Vietnam you'll find a lot more "hard sell" but thats not a defect of the country - simply people trying to earn a living in a place with a lot of competition. Avoid the worst of the backpacker trail, (which might be tough on a really tight budget) and you'll avoid a lot of this
In Cambodia you'll get a lot more of the offers to do things legal, quasi legal, outright illegal and/or just distasteful ( drugs, child prostitution and the like) maybe due to the crushing poverty that most of the country lives in. If you feel like doing some volunteering this certainly is the country to go do some in.
Lao is the least stressful of the places in terms of those kind of things and you'll find a people who are generally very happy and friendly, but perhaps not quite as outgoing.
All of them have beautiful and really ugly places but I've loved visiting them all.
Don't think you can go wrong with any of them.