With help from Cineform 4K ($1500), Premiere Pro can even do Real Time editing.
In Vegas, you'd have to edit L & R separately and then assemble in After Effects or equivalent (I think there are some open source tools that can do this also).
I really need to increase the Vegas project and rendering size limits.
I know how to write Vegas scripts, can it be done by script?
Are there registry settings or ini files that can be changed?
I don't think there is any way to increase Vegas resolution limits, those are likely to be totally hardcoded.
What's your presentation device to present the 3D?
Most presentation devices use square pixels, and that means that to get the best quality you really should switch to 3840x1080 with square pixels (PAR 1.0) before doing anything to the video.
To work in Vegas, you could just keep the L& R channels separate, render each to 1920x1080 at PAR 1.0 before doing anything else to the video, then pick one channel to edit, do the work, render the output, save the .veg file and then apply the .veg to the other channel.
In Hollywood, they have the advantage of being able to monitor in 3D, and their compositing tools understand 3D.
The open source tools such as Avisynth can do just about anything, but never with ease. Major complexity.
For ease, it would probably be better for you to buy an old, cheap version of After Effects just for the simple task of combining,
What is the output format for your presentation? Is it really 2 camera frames side-by-side in one projection frame?
Vegas 8.1 does support 2880x1080 projects, 8.0c does not!
You must create a custom template to render greater than 2048.
In the custom setting dialog select the video tab.
Then set "Image Size:" to (Keep Original Size)
This worked for "Format:" Windows Media Video V9 Advanced Fornat
Have not tried other Renderers to see if they support large formats.