Now that makes me suspect you are downloading the file as a video instead of the music only. This is common when using "free youtube downloaders." It may display as a blank screen (video) and play the music but iTunes would not see that as a MP3 file. Tagging it MP3 doesn't make it an MP3. For example, grab some image jpeg and try to rename it with .mp3 at the end. It will work but that file is very obviously NOT an MP3.
I don't know the functionality of mp3juice. Youtube downloaders will sometimes let you choose to download the audio only or download the whole thing. I looked it up and their site shows this: "allows you to download songs as mp3 (audio) or mp4 formats in HD and Full HD based on quality availability." You might have a setting to toggle to choose audio only. Has it worked (went into iTunes) with ANY other audio you've downloaded? If they all work in VLC but won't import into iTunes, the fault it likely in mp3juice... and probably only a setting.
VLC is basically swiss army knife. It can open things well beyond Apples codec range. Again, did you try opening it in Quicktime? If you open it in Quicktime, does it play there? If it plays in Quicktime, it should load in iTunes. If not, File, Export, Audio Only to export it as an audio-only file. If it is technically a video file now, that will strip off the video portion. If exported from Quicktime to an audio file, iTunes will almost certainly import it.
If you don't have access to Quicktime, you might
try the same export approach with VLC