Can I connect a second monitor to the VGA port?
Most likely yes. It depends on the design (and the GPU) of your laptop. I never saw a recent (as in build in the last 6 years) laptop with two video outs where only one was usable.
Will their be any real diference in image quality?
Again, it depends. The hardware in your laptop generates a digital image. For VGA it needs to convert this to an analog signal, transfer it over a cable and then let the (LCD) monitor reconvert it to digital information. Usually there will be some loss here.
Having said that, VGA can be decent, even at higher resolutions. But not with an el-cheopo cable. There are lots or marginal or simply poor quality cables out there which means that on average VGA on a modern display will result in poor quality or ghosting.
DVI, HDMI and display port are all digital standards and do not suffer from this extra conversion. They should all result in a better image than via your DB15/VGA connector.
In case I only want to connect 1 monitor - is it better to do it via HDMI or VGA?
HDMI, for the reasons stated above.