Performance of Virtual Machines on MacBook Pro with and without discrete graphics I am trying to decide between MacBook Pro 15" with and without Discrete Graphics Card. My work involves running 2-3 Virtual Machines using Virtualbox 
at a given time with Chrome, Microsoft Outlook &amp; Skype running on OSX. I am not in gaming at all.

I already have a MacBook Pro 13" Early 2011 with 16GB RAM and 256 GB SSD. But this machine gets very hot when I run above tasks on it. 

At my location MacBook Pro 15" with Graphics Card and 512GB SSD is going for $2499 and MacBook Pro 15" without Graphics Card and 256GB SSD is $1999. So for $500 I am getting extra 256GB and Discrete Graphics card. Is $500 worth this upgrade for my workload.

Also next machine I want to buy is in 2020. Please help.
I run three ubuntu VM, light garage band or Logic Pro and Safari, FaceTime and Apple productivity apps on the 2015 MacBook - base model, base storage, base RAM.

You'll have significant CPU hogs choosing Chrome over Safari (in general), Skype over iMessage and Office over Apple's Apps. 

Whatever Mac you push with that workload will become thermally overloaded and throttle the CPU, and since you want the Mac to last for half a decade, you might as well get the best CPU, most RAM and go for the 15 inch model.

You could run all the apps on a lesser Mac, but if you don't care how hot it runs, get the most watts you can pump out and you'll have the best performance.

You want the 2.8 GHz CPU, so you get a better value with the dual GPU option since you get another 2 GB ov VRAM which will get used / free up some of the 16 GB main memory for other uses.

If you doubt the 2.8GHz Quad-core Intel Core i7, Turbo Boost up to 4.0GHz is enough, you should get a lesser portable and run your VM on a desktop even if said desktop is a Mac Mini.

You also might consider getting a MacBook / 13 inch and combining it with an iMac for the cost. Much better VM, but you don't carry it all with you. Remote access to a VM isn't any slower than running the VM locally - in fact, it's probably much faster with a desktop CPU instead of a mobile CPU where much more energy can be had, much more heat dissipated and more bang/$.

Good luck buying. It's an interesting CPU and RAM centric workload, so you have lots of good tradeoffs to make - every $ you upgrade will give you noticeable benefits, so it's really what your time is worth and how much sacrifice you'll endure to buy the top of the line or two Mac to run that workload.