The chances of that happening are low anyway. I believe someone reported their server was running smooth for 3 years with only a single _minor_ downtime. Also, I doubt that an app that runs on a VPS is as important as one that requires a whole dedicated server. But yes, if a box happens to fail, how slim the chance may be, then your site is offline, just like Amazon EC2 that was offline for 3 days iirc even though they have multiple availability zones per region.
Also, running 3 dedicated servers for redundancy still isn't expensive when you compare it to traditional VPS/EC2. However, you do have to do everything yourself of course. Tbh I feel that people always tend to worry too much about "downtime" or "failing boxes" while the reality is that most people don't ever experience huge downtimes. If the app is really _that_ important then you're likely making some good money to justify paying for more expensive solutions and there wouldn't be a reason not to do that.
Also, running 3 dedicated servers for redundancy still isn't expensive when you compare it to traditional VPS/EC2. However, you do have to do everything yourself of course. Tbh I feel that people always tend to worry too much about "downtime" or "failing boxes" while the reality is that most people don't ever experience huge downtimes. If the app is really _that_ important then you're likely making some good money to justify paying for more expensive solutions and there wouldn't be a reason not to do that.