For downloading Twitch VODs, I use Twitch Leecher which can be found here: https://github.com/Franiac/TwitchLeecher/releases

For downloading YouTube VODs, I use youtube-dl which can be found here: https://youtube-dl.org/

youtube-dl is really powerful and can download VODs from a ton of sites (including Twitch). The one downside is that it doesn't have a graphical interface, so you have to use it on the command line/windows command prompt and it can be tricky to understand if you're not used to using programs that way. But it's not too hard to use or figure out, you basically just need to type "youtube-dl URL " and then put the URL of what you want to download and it'll download it to whatever folder you're currently in. It's got a ton of options if you want a specialized run, but in it's simplest form, it's just that. I recommend reading the documentation though.

Another tool that can be used for both services is JDownloader. Then open with Avidemux or Yua to time.

I used to use the Twitch Leecher but it did some odd stuff to vods sometimes.

I would download a vod, time exactly 60s of livesplit running, but I would get less than 60s in frametime. I assume it was just smashing the video together in any points where frames were dropped, but I have never bothered to find out if that was the exact cause.

I've used JDownloader2 since, and I've never had any issues with it. For pulling YT vids or Twitch vods. So I can recommend it