My guess is that you’ve left all the various folders at their default locations which is why things are getting full.
The Default and Temporary Files folder locations are found in Options-Preferences-Burning tab.
Find out where yours is and clear it out.
If you have a second drive for editing purposes (highly recommended), create a new Temp folder on this drive and change the above folder locations to point to this new one.
Do the same thing in Vegas as well with it’s Temp folder locations.
As far as your file size, have you confirmed that it is indeed 8.9 GB. in size?
DVDA has a long history of incorrectly reporting file sizes.
If you know that you’re under the limit, ignore the warning message and DVDA will proceed as normal once you press ‘Next’.