So no one found the most reliable answer of above question, even after contacting Oracle people and googling too much.

The question remained unsolved since 2 Aug 2011 in official Virtualbox Forum : VirtualBox as a VPS Hosting platform?

So what is the most simple and most trusted answer? I am going to answer today:

The Simple and most trusted answer: RESOURCE POOLING and DNS complexity

The most precious thing is RAM , Cores and Memory. When you use virtual box it uses your HOST machine RAM and it strictly allot the same to each virtual machines.

For Instance, you have 12 GB RAM Host and 500 GB HDD and you need to create two VMs hosting two servers Ubuntu Server1 and Ubuntu Server2.
This is what happening :
Virtual Machine 1 : Ubuntu Server 1 : 4 GB and 200 HDD
Virtual Machine 2 : Ubuntu Server 2: 4 GB and 200 HDD
HOST Machine Left with : 4 GB RAM and 100 HDD

In above example note that Virtual BOX will fix the RAM for each VMs (servers) and even when VM1 is not in use will not allow other VM2 to use its resources.
Virtual Box doesn't allow resource pooling of RAM, cores and Memory and only limited to HDD pooling. Even swapping of HDD to RAM will not save you because its uses core RAM not reserved.

Further, your Host Machine should not crash because of fixed resources (RAM) allocations to VMs. If this happens then your Virtual Box will also crash resulting crashing of everything.

So can I use in Production? For small Projects yes (if you don't care about pooling) and for Large projects NO.

------------------------------ A HACK ---------------------------------------
Remember my post about YOUR COMPLETE ONLINE DESKTOP: Hosted on Server in just $5.00 :).
Go ahead and create a desktop on server.
Then Install Virtual box there and go Crazy, Crazy, Crazy....
------------------------------ A HACK ---------------------------------------

I am not a Programmer. I love Programming and I am among 17 % of World's programmer population who do not hold a formal degree in Computer Programming. I am an Engineer (Chemical Tech -Food Techno) who knows more about Heat Transfers and Mass Transfers and loves exploring.
Treat above stated introduction as my disclaimer that I can't answer all your queries. But I can Mostly. Contact me on manishfoodtechs@gmail.com

DON'T FORGET TO LIKE, TAG & FOLLOW ME. In coming posts I will explore creating your own compute clouding server like Digital Ocean and Google cloud. Yeah... Interesting...