Sadly I don’t think such a beast exists, or could exist. A microDIMM’s thirteen multiplexed address lines (A0-A12) aren’t physically capable of accessing more than 256 MB.
Larger microDIMMs are theoretically possible if they allocated another address wire, but your VAIO wouldn’t be wired for it.
I really do hate to be such a killjoy because my C1MW also uses microDIMMs, and it sure would be nice to expand it beyond 384 MB!