just asking by

Steam cloud sync. if not, search on google for its save files, find your files, copy on pendrive or backup somehow.

Use GSM. You can keep backup of all your gave saves in a flash drive or upload to Dropbox etc.

Im not sure if new vegas has steam cloud support, but you can manually backup the save files by going to
"C:/Users/[YourUsername]/Documents/My Games/FalloutNV", backup the files to another location and replace it once you have a fresh OS install.

I use GameSave Manager to back up all my saves when reformatting and whatnot.

It will find all your Steam saves, but you can pick and choose which ones you want to back up. It will even back up your GFWL profile which might come in even more handy once the shit hits the fan in July.