Encontrei uma pequena solução que publiquei na pergunta duplicada do stackoverflow lá . Um virtualbox [bug] [2] está basicamente por trás disso tudo. Citando a minha própria resposta do stackoverflow, se você não quiser ir no site:
Well since the the ZAxisMapping should already disable the horizontal scrolling in my question, but of course did still not seem to work, I have found that it was actually a virtualbox bug that has something to do with the ALT key modifier. Basically the scrolling will switch from vertical to horizontal if the ALT key is pressed at the same time as falling into the virtualbox window. This happened to me because I was using dexpot in Win7 with the desktop switching feature using the alt key.
Until the bug is corrected, the workaround is after getting into the vbox window and noticing that scrolling was switched to horizontal, you need to hit the host key, hit ALT again, then manually click the vbox window to return to it, without touching the alt button. For now, I do not use the alt key for switching desktops, and everything works fine again.