The other thread didn't tell me if you already checked that you're using the correct driver (i810 if I read that right; check it from the thread you linked first). Are you?
grep Driver /etc/X11/xorg.conf
one of the lines should return i810, or possibly some other driver being used, but if it returns vesa driver you'll need to change that. Changing the driver means opening xorg.conf as root, locating the correct Driver line (in the section that describes your video card), replacing the driver name there with a correct one (note: lowercase is probably different from uppercase, so be careful) and saving it. For example
sudo gedit /etc/X11/xorg.conf
and then find the line and change it. After this you should make sure each of the resolution lines have the preferred resolution as the first resolution from the left, like you seemed to have already. Then restart X with CTRL+ALT+BACKSPACE and see if that helped.
Yes, it's a Sony, and it's a Vaio - but it still should work. Before ramming at odd programs like '915resolution' you should always check the very basic configuration like video driver being used, to make sure every general setting is in order. To use a specific resolution your card must be able to do it (probably is, you said it's "native" resolution) and your video driver must be able to do it.
I'm not sure about the driver I mentioned above, just took it from the other thread. If it doesn't work, search the web (or Sony site) for information about which chipset you exactly have (or use command 'lspci') in that laptop, and then what (Linux) driver should be used for that chipset.