Are you sure that it is using VESA? Your xorg.conf shows it should be using SIS. Try sludo lsmod | grep sis.
Anyway, here are a couple of things that may be relevant:
Identifier "Generic Monitor"
You may need to adjust the vertical and horizontal refresh rates. Setting them too low may result in low maximum resolution. Setting them too high, however, will probably end in a blank screen so you should try and find the exact refresh rates for your monitor (manual, google, ...).
Identifier "Default Screen"
This one says 800x600 so it obviously isn't going to get any higher. You may want to add something higher - although success may depend on adjusting the refresh rates first.