A USB to VGA is essentially its own graphics card and usually not a very powerful one at that, since VGA usually requires more bandwidth than USB can handle to work properly. For that reason, they're usually only used for projector presentations and the like.
Generally, no. USB is not the magical speed-all answer to everything; but if you're lucky enough to have some USB 3.0 ports, it might be just enough to handle the bandwidth required to run some of the lower-end games -- but if you're running 2.0 or earlier just forget it, it ain't gonna happen.
Video games of today require a high transfer rate to work smoothly, and there's no way any older USB device can handle that. There are USB devices, however, with accelerated GPU's, costing around $60 or more (at least when I looked just now), and they all are USB 3.0 and up. That does not mean they won't work on a 2.0 - but for what you need it for, you should look before you leap - if you have USB 2.0 ports, just get a new graphics card instead, it's a lot cheaper and more powerful.
It sounds awfully convenient but it definitely is not practical given the restrictions that USB has. No matter what USB ports you do have, you will still always be better off with an internal GPU. A good analogy would be this - imagine that your PCI-Express slot is a giant utility pipe carrying gallons and gallons of water into your home. Now, try squeezing all those gallons through a single coffee straw. That's USB.