Just remember that there's nothing magic about VPSs, although you would think there was to hear some people talk. You have just introduced the overhead of maintaining 5 individual installations and for what?
I'm surprised that the VPS solution is cheaper than one box. I'm more than prepared to be wrong but are you absolutely sure you are comparing apples with apples. That's guaranteed 2GB RAM is it? And what's the processor doing - do you have any guarantees about that?
Hardware and performance
500 concurrent users through multiple vps-s?
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