Did you get a new computer?
Vegas uses the fonts that are installed on your computer.
If you want more, you'll have to either copy them from another computer or download them from a site such as
http://simplythebest.net/fonts/free_fonts.html
To install a new font, go to C:WINDOWSFonts, File - Install New Font and follow the prompt.
Once that's done, it should show up in Vegas.
I find it useful to put the fonts to be installed in a folder on my desktop and then browse to that folder.
After they're installed, delete the folder as it's no longer needed.