There is no rule that says you have to use Prores or DNxHD or any other intermediary codec. I have yet to see any good reason for it, and in all my testing, I have yet to see an advantage. I avoid intermediary codecs like the plague.
I know this goes against the grain of ‘common knowledge’, so instead of justifying it to you, I’ll recommend that you shoot a few seconds of footage and test for yourself to see if there’s a substantial difference to warrant it.
I shoot and edit native, with just one render between source and master. If and when I need more ‘data strength’, I prefer to do it the best way – which is via TIFF image sequences. Vegas can easily edit H.264 native.
Regarding the issue with speeding up in Vegas, I don’t have a specific answer to give. Have you read the manual? Most NLEs spell out clearly how they handle frame rate conversions. Maybe somebody with specific Vegas can chime in.
https://www.wolfcrow.com – Workflow information and support for filmmakers, photographers, audiographers and videographers.