I want to buy a new Monitor for Rpi. I don't have budget for HDMI monitors not even for powered HDMI to VGA converters. So can I use vga to usb data cable and use it with powered usb to connect display with Rpi 2.
There are varying adapters available that you can use on the Pi:
- HDMI → DVI
- DVI → VGA
- HMDI → VGA
- composite → VGA
- composite → SCART
You can get a list of verified peripherals here - these include and recommends externally powered adapters, but also includes some (cheaper) non-powered ones, as explained (this may concern only the old Pi models):
Note that active converter boxes may draw power through the HDMI port, and thus will put an extra load on your PSU, and also increase the current running through the Raspberry Pi's primary input fuse. HDMI ports (and the raspberry PI) are designed so that they deliver a very limited amount of power (50 mA) to the TV/Monitor/display-adapter and much more isn't in theory allowed. In fact there is a diode (D1) in series with the power line which can only handle 200 mA, if the adapter tries to draw much more than that the diode might fail. Therefore only externally powered adapters are to be recommended. Despite this, many people report success with non externally powered devices. If you have bought a non externally powered HDMI to VGA adapter, and you experience problems with it (It behaves badly, D1 burns out, F3 "blows", or your PSU overloads), then not all is lost, there are cheap (a few dollars) adapters that allow you to add external power to the HDMI cable! An example can be found here: 
The HDMI adapters require power (here (from here) shows pin 18 needs +5 V (min. 0.055 A)), which usually should give 0.55A whilst adapters may need more power (VGA spec apparently does not specify).
This is why the page shows that you can use this, though it does show you can use a few non-powered converters work here.
You can also use the composite video output (read here for newer versions), another alternative is to use a RDP or VNC server and a remote desktop client on another machine on the network to view what is on the Pis screen.
As per this Raspberry pi Forum thread, even if you use a USB to VGA cable, it would be eating up a lot of RAM.
That link sounds promising but it is not out yet. Ie, Rpi foundation is still working on it on the software. But in that case, you won't be able to use much of Rpi GPIOs as it is mentioned in that link.
I guess, HDMI to VGA cable should be a decent solution if you already have a VGA monitor.
Hope it help.
This screen will eat 43% bandwidth(~19MB/s) of a RPi 3B+ USB 2.0 Port(~44MB/s)when it works under a 800x480 25fps resolution. The RPi CPU utilization is low than 15% as almostly all jobs done under RPi VC4 engine.
Looking at the specs of a usb video adapter at BestBuy, it seems the units have their own onboard DDR ram.. so I can't see how it would eat up the Pi's ram. It would utilize it's own ram. It would be worth the shot. How to get the Pi to use it at boot might be a little difficult though.