Which videos am I allowed to use on my covers?

I was planning to do some vocaloid covers. I see that on the original videos in NND, the producers post links to the instrumentals. This means that I am allowed to post a cover right? However does that mean that I am also allowed to use the song's video? If not, how can I tell whether I am allowed or not to use the video in my cover?

Generally, I think most Vocaloid PV's are free to use if you just credit properly!

I see, thanks.

I was planning to cover Just Be Friends, however, I read that there was an issue some time ago with the copyright of the animations used in the video which caused the original video in NND to be set to private.

Do you know if it would be OK to use the original video form Just Be Friends in my cover?