You can add 2GB in the empty slot certainly.
It may even be possible to add a 4GB stick but I can't say for sure as I can't find any specs or upgrade info for that particular Vaio model.
I'd be tempted to buy a 4GB stick from a vendor who will exchange it just in case.
It must be a bit of a pain trying to run a laptop on only 2GB RAM I must say - -