recovery?
the only thing i know you can do from power off is enter recovery mode, connect your vega to a pc by a usb cable..
Hold the power button, and count 6 seconds, after 6 seconds the screen will blink off, at this exact point release the power button and begin holding the back button for 6 seconds, after those 6 seconds keep the button held but hold the power button also for 2 seconds, so both are pressed, after those 2 seconds you should see the safely remove icon pop up in your startbar (like when you plug in a usb stick) and then let go of back, wait a second and let go of power
NOTE THIS WORKS ON ALL ROMS
at this point the screen will be blank but if you unplug every usb device but the vega cable, and the safely remove is still there then its in recovery.
if you havent used recovery mode before then install the vega recovry drivers off the myadventvega website by opening device manager, clicking your vega (should have a yellow triangle with a ! next to the icon) and click install driver, choose not to search online, say you have a CD or files, and point it to the folder which you downloaded from myadventvega [NOTE NOT THE FOLDER INSIDE THE FOLDER YOU DOWNLOADED< JUST THE FOLDER WITH TWO OTHER FOLDERS INSIDE IT.}
after you are free to download the rom of your choice (The nvflash mode) and run it to install it