Customize toolbar of VLC On Windows/Linux, it's really easy and straightforward to customize the VLC toolbar by using the menu "Tools">"Customize Interface..." (See this help page.)

Somehow this menu doesn't seem to be available in macOS.

Is it just a missing Mac functionality, or is there a way to customize the buttons showing on the toolbar on macOS?
It is indeed missing functionality. You can add (but not change the positions of) some media play buttons like Previous, Next and Show Equalizer to the interface toolbar using Preferences... under the VLC Menu. Clicking the Show All button which appears at the lower-left corner of the Preferences... window will enable a great deal of additional customization, but it's challenging to anticipate or even recall which features are available in Basic, Show All, or both modes. 

I'll alert you to a potential hazard: It's mandatory to save any changes you make to VLC's prefs before moving from one preference area to another, much less between Basic and Show All modes. Otherwise, your changes won't be retained--and there's no warning this is the case. Plus, annoyingly enough, the prefs window closes when you do save your changes, and re-opening it always opens in Basic mode. Furthermore, changing an item in one mode can cause changes to occur to seemingly unrelated items present the other, unseen mode--again, without warning.

Also, as far as I know (unless a recent nightly build has re-enabled the option), skins aren't customizable in MacOS and haven't been for years. Oh, here: Let me save you ten minutes of wandering around clicking disclosure triangles looking for the additional media buttons for the toolbar in the Playlist. They're here: Preferences ▸ Show All ▸ Interface ▸ Main Interface ▸ macosx ▸ Appearance

Mind you, if you're determined enough, the Main Interface section is also where you would go to run VLC on a homebuilt Lua or ncurses  interface--and put your buttons wherever you please.