This response may be a bit overdue but I just found this forum. Our company has alot of experience with VPI windows. We have installed several thousand of them over the last two years and I must say that they are by far the best residential grade vinyl window company in the NW. We test on every project and these windows have never failed, in fact they consistently perform above spec. They have great customer service and stand by what they sell. They also have very good techincal knowledge when questions arise. There are more expensive windows with higher performance characteristics such as Innotech. But for the majority of low rise buildings VPI fits the bill.
What do you test on every window job?