Youtube supports up to 8k these days.
Your smoothest HD codecs to edit in Vegas will be HDCAM-SR-Lite, XDCAM422.MXF, XDCAM-EX.MXF, or Cineform.avi. XDCAM is a highly optimized workflow in VEGAS utilizing the GPU well for both rendering and Timeline playback.
If you edit or render out in XDCAM format, you can render to xdcam and directly upload the file to youtube.
This all assumes your system is powerful enough to handle smooth HD playback in the first place.
You could also try converting your 3K to cineform.avi and see if the playback is good enough. Make to use the Preview half or draft half settings on the preview for editing, and Best Full only to see effect results or CC. That will help with playback issues the most.