That's a really different boat- it's double the weight so at least double the cost, sails 4 up instead of 2, etc. It'd be just as rational to compare the VX to a Viper...
The VX is really a nice boat, well made, well thought out, and fun to sail. You know, you just write a check for an amount that is not too high, and hit the water planing away. No year and a half of work before ever getting wet. No breathing fumes and clogging up the lungs and killing brain cells. Just splash it and go with a big grin.
Viper's fun too, just an older design.
I have raced on Vipers some and they are defiantly a super fun boat to race. So why build a boat like an i550 instead of buying a Viper? Well for one its less expensive and i really enjoy the process. I have learned allot: Boat design, Foil Design, Composites, How to work with lead. I think in the end the process of building a boat from scratch and figuring out many issues and different options i will be a better sailor for it. It defiantly wont be the best one design racing in the world when its done. But how cool will it be to race next to someone with a straight sprit vs an articulating sprit and see which is better in what conditions. or someone with a jibing keel. It will be interesting to see how a complicated hard to sail boat competes vs a simple easy to sail boat. Years in the future it will give me ideas when setting up another boat what things work and what dont so i will be able to maximize the performance of whatever boat i am racing to the extent of the rules.