@iEmby The way I've done it in the past is to shoot at a double frame rate (eg, 60, 59.94, or 50) and then turn it into slowmo with ffmpeg by just patching in the rate divided by 2 without resampling. The operation is a very quick repackaging with nothing but a different frame rate in the header. The clip does not change much if at all in size. Here I changed 59.94 fps to 29.97 slowmo:

ffmpeg -itsscale 2 -i "MVI_0068.MP4" -c:v copy -c:a copy "MVI_0068_slo30p.mp4"

Then bring the slowmo clip into Vegas. I usually do this when I want to keep it as a smooth slowmo but you can restore the original playback speed in clip properties by doubling it. If sound is important, you can also pull in the original clip and mute or delete its video track. But as I recall when I did the above operation, the original audio was unchanged in the new clip, but only extended halfway through it because the video length was doubled. So if you only double the Vegas video stream play rate, the audio should end up back in sync.