My first guess is the keyboard emulation vorpX is using.
I’ve noticed that when I type things into game chat when using vorpX, some keys are remapped. For example my K key delivers a German umlaut U, and some special keys like period or question mark don’t display.
If this isn’t just an issue unique to me, you might be able to get around it by temporarily enabling an AutoHotkey script to remap the keys for your login.
But I’ve never tried ESO, so we’ll still need a proper response from Ralf to know for sure.