Sad to hear! I noticed that some are available through the Internet Archive:

https://archive.org/details/Hexaflexagons

That's heartbreaking to hear. I wonder what happened to vihart. Her videos were so creative and fun.

Seems she's moved on - now at Microsoft https://en.wikipedia.org/wiki/Vi_Hart those videos were such fun, shame they're not there anymore.

nooo i hate this is how i found out. i miss the hexa flexa mex. she definitely influenced how i would doodle mathy stuff in class growing up