So I have both 4k 30fps and 2.7k 30fps footage. I plan to downscale or crop to 1080p. (Most footage will simply be downscaled).
Which workflow approach is best for highest quality and/or efficiency?
1. Drop 4k and 2.7k footage into 2.7k or 4k project and use proxy files, then render as 1080p
2. Drop 4k and 2.7k footage into 1080p project and render as 1080p
3. Downscale 4k and 2.7k footage to 1080p using seperate project in Vegas, or other software, then drop into 1080p project
4. Any better suggestions
The answer partially depends on how capable your system is. You'll just have to see what works best for you. In any case, I'd start with your option 2--just drop in onto the timeline and see how well your system handles the footage. If your system can't handle the footage directly, then create proxies for your editing. V13 can do this directly or you can use third party scripts such as Vegasaur. I would NOT go with option 3. The primary advantage of using 4K or 2.7K footage is to be able to crop your footage if necessary. If you downsample, you lose this capability, plus your final project will lose some quality by the fact that you've now rendered twice.
I would ask what your final display format is first.
As Wayne suggests, if you downscale everything you lose the ability to crop in on the 4k footage with no loss. However, editing at 1080 in HDCAM-sr-lite or XDCAM422/ex will be much easier on your system if the project is complex.