Sound Forge Pro includes the legacy Direct X noise reduction NR-2.0 suite which has 'Clipped Peak Restoration'. iZotope's RX Elements (VST) is included as well which basically has the same features but newer/better. SF can edit down to the sample level. Once installed, both NR-2 and RX can be used directly in VP. Magix's Sound Forge Audio Studio or Audio Cleaning Lab likely has something at a lower cost Analog stage clipping and severe digital 'overs' are not so easy to fix and often impossible, even with RX Advanced ($$$) or Spectral Layers, so YMMV
Yes, although it doesn't need exporting. Vegas doesn't touch the original mp3. You can (while Vegas is open) edit that mp3, save it under the same name, and when you switch focus back to Vegas it detects the file has changed and rebuilds its "peaks".
The scenario where exporting the sound track out of Vegas would be smarter is where you edited the 2hr source down to say 8 mins (pretty likely actually) and then you will be clip fixing just an 8 min mp3. I could actually do that... now that I have written this :) A lot of my finished jobs are 20-30 mins so still over 10 mins though.