VGA is an nvidia geforce GT120M, and I'm using proprietary drivers.
When I try to put laptop in stanby mode it wakes up immediately, except when extremely low on battery [in that case it wakes up immediately after i plug in the power cord].

http://en.opensuse.org/S2ram

You may wish to try the s2ram tool described at the link above. Your particular machine is not in the whitelist as far as I can see, however that doesn't mean you can't use s2ram to suspend it, and/or to discover why it will not suspend. There are lots of other Asus machines in ther, and one of their s2ram configs may work for you too.

That s2ram page describes common problems that occur with suspending too.

I use a desktop machine, which suspends fine ONLY IF my USB controller module (ehci_hcd) is built as modules, not built into the kernel; I need to remove my USB module(s) before suspend, and re-insert them after resume, for it to work correctly.
Also, (on my machine) if my kernel has the "Hangcheck Timer" enabled, that will also cause my machine to resume immediately on the second or subsequent suspension.
I have not found that the nvidia module impairs suspend, but it may vary depending on the module version.

Also, check out the text files in the kernel source /Documentation/power folder -- there's lots of info in those files.

Best success,
Sasha