Comment 7 for bug 59618

In fact.
The debconf script of xserver-xorg, when executed, sees that "xforcevesa" flag is in use and sets vesa as the default driver.
So far so good.
But then it makes a probe of the video chipset, and if it finds a known one, overrides the default (vesa) with that one (in my case, sis)!

The solution would simply be to modify the debconf script so that, if xforcevesa is set, the probe is completely skipped.