I am pretty a new comer to Appscale and GAE. Say I've got a web application on GAE (offers hassle-free maintenance on server security etc), basically move the applying to some Xen VPS host using Appscale, will i still enjoy hassle-free maintenance (since Appscale is dependant on GAE)?

No. GAE is a couple of things - a platform, along with a service. Advertising media are your applications on Google's GAE infrastructure, Bing is supplying the service of keeping the infrastructure ready to go. As a swap, you have to pay them (only when you utilize a lot more than the free quota.) If you opt to install Appscale by yourself hosts (does not matter when they are virtual or otherwise), you'll are in possession of the task of keeping things running. As a swap, it will save you the cash you could have been having to pay to Google.

This really is the same as should you have had just one HTML page you desired for everyone - you can pay a website hosting company to host it for you personally, straight forward, or you might install IIS or apache by yourself computer, and serve it yourself.