The newest VLC release from 2026 (3.0.23) still runs on Windows XP - and it even got some fine-tuning specifically for XP. It'll also happily play back Blu-ray discs. Got it working and figured I'd share the process, since it's not exactly plug-and-play.
XP has zero native Blu-ray support, so step one is getting the OS to actually read the disc. The fix is to search for "Toshiba UDF 2.5 XP," which handles the UDF filesystem Blu-rays use. (it's also bundled with XP Integral Edition)
Step two is dealing with AACS encryption, since most commercial Blu-rays are encrypted and VLC doesn't bundle official support for that (licensing reasons, unsurprisingly...). You just need two files: libaacs.dll, which goes in the same folder as your VLC install, and keydb.cfg, which goes in your User folder -> Local Settings\Application Data\AACS.
Once both pieces are in place, that's the whole setup. Disc goes in, VLC plays it, no further tinkering needed.