I got a clip of the issue. Here is a google drive link.
2020-10-03 16-50-11.mp4
The first time I clicked the void vault, I walked forward so I was not touching the portal and had smart cursor off. I had potion of return selected. It teleported me.
The second time I clicked the void vault, I stayed on top of the portal and had smart cursor off. I did not have potion of return selected. It teleported me.
The third time I clicked the void vault, I stayed on top of the portal and turned smart cursor on. It did not teleport me.
The fourth time I clicked the void vault, I stayed on top of the portal and turned smart cursor off. It did not teleport me.
It does not seem like the item I am currently holding effects the outcome. It might have something to do with where the portal comes from, but I doubt it, as I have had this issue occur when teleporting from many places. For some reason, smart cursor fixed the issue, which means that it might be related to what you said about clicking nearby the portal.
If you are not able to replicate the issue, I could do some real tests to try and determine what the issue is.