I would like to properly license a Helvetica Neue font to use on a website. However, I can't figure out who owns the font, or who to buy it from, and whether the people selling the font online are its legit owners. Does anyone know how this font is sold (is it legally being sold by multiple vendors?) and who to buy it from?
Your question is tricky, because even the bad people will put "guaranteed not stolen" on their website. If you carefully edit your question, so that you are not accusing, you could show us a link of the vendor(s) you have questions about - and the community shall have trust or no trust for you.
Here is my personal way of "knowing":
Please go to an established and trusted vendor. For example I take some of my very humble budget to myfonts (not affiliated in any way):
"Helvetica Neue" is just another way to say "Neue Helvetica" which would be its real Swiss Name. Putting the adjective behind makes this typeface show up next to "Helvetica" in catalogs and online-shops. Read the blurb on myfonts about the history of this typeface and its several reworks.
If you still hesitate to trust Linotype, read on here: https://www.myfonts.com/foundry/Linotype/