If I was doing this, I would convert my 50i fx7 footage to 29.97p lossless intermediates and bring them back onto a Vegas timeline to mix with the GoPro footage and then later render out for the web. I have developed a workflow for this that uses the magic of MVtools and QTGMC in AviSynth. It gives excellent results but is extremely complicated if you're not used to these tools. Have a good night's sleep and a strong cup of coffee before you even think about embarking on this.
Get your 50i fx7 footage on your timeline and follow this tutorial. But you need a different AviSynth script from the tutorial in order to convert to 29.97p instead of 25p. Mine looks like this (single-threaded version):
AviSource("d:\fs.avi")
AssumeTFF()
ConvertToYUY2(interlaced=true, matrix="Rec601")
QTGMC(SubPel=2)
super = MSuper(levels=1, pel=2)
MFlowFps(super, QTGMC_bVec1, QTGMC_fVec1, num=30000, den=1001, blend=false)
With multi-threading code added and optimised for my particular computer, it looks like the following (but you might want to get it working single-threaded first):
SetMemoryMax(768)
SetMTMode(5, 4)
AviSource("d:\fs.avi")
AssumeTFF()
ConvertToYUY2(interlaced=true, matrix="Rec601")
SetMTMode(2)
QTGMC(SubPel=2, EdiThreads=2)
super = MSuper(levels=1, pel=2)
MFlowFps(super, QTGMC_bVec1, QTGMC_fVec1, num=30000, den=1001, blend=false)
Instead of encoding with x264 in MeGUI, render out a lossless .avi file. So after section 3.3 of the guide, skip to section 4.6.1. Instead of Lagarith, I have started using UT Video Codec for my lossless intermediates because it drops fewer frames during preview in Vegas. i.e. it plays back more smoothly. Be warned that lossless .avi files are HUGE so you need lots of disk space. Also the AviSynth processing is very slow so do as much cutting of unwanted footage as possible first in Vegas.
If you have trouble with UT Video Codec then use Lagarith lossles or try the near-lossless GoPro Cineform codec.
Once you have your 29.97p lossless file you can mix that one the timeline with your GoPro footage. Be warned that Vegas can sometimes give you black frames or black clips on the timeline, and this seems to be a bit more of a problem with files rendered with the type of codecs I have mentioned. Restarting Vegas tends to fix the problem so I recommend doing that before any final render.
When you're ready for your final render for the web you can either use the Sony AVC or MainConcept AVC codecs in Vegas, or if you want better quality for the same bitrate you can render to x264 in MeGUI as per my guide. In that case you would just open up the frameserved fs.avi file in MeGUI since no further processing would be required by AviSynth.
If you can't face any of that and want to do it all in Vegas, experiment with changing "smart resample" to "disable resample" in the video event properties. That changes whether Vegas blends or repeats frames, and might give you a better result than you're getting by default.
