As to my knowledge, Bramble Vest and Thornmail should not cause you to receive tower aggro, but perhaps this has changed recently and I missed it. If that did NOT change, however, then there is a bug. I have found myself being aggroed by the tower in several games when an enemy champion procs my bramble vest or thornmail. I am not wearing sunfire cloak or doing any damage to them other than the reflected damage of bramble vest/thorn. I am not 100% certain if I have been using it in every situation, but in the most recent instances I have been running the Demolish keystone. This has happened to me playing both Dr. Mundo and Nautilus, so I do not believe it is champion specific. Other than Demolish I was also running the Conditioning and Second Wind keystones in most, if not all, the instances it happened. Assuming the items themselves haven't been touched recently, I would suspect the issue lies somewhere in Demolish, since it is a tower related bug.
Hope this helps and you can get a fix out there!!
A Dr. Mundo Main
Edit: Just thought I'd add that I did end up doing some testing with the item against an intermediate bot in the practice tool. I took aggro regardless of having Demolish, Second Wind, or Conditioning.