As for the Vista driver for the R300, it should work no problem. According to the Epson website, the R300 driver is (should be) embedded within Vista.
I have an XP laptop, Desktop and a Vista desktop. All are networked together and can share files (with the occasional fit thrown by Vista). Now my printer is an R200, but like yours the driver is described as 'in-box' or part of Vista. I have my printer attached to a wireless print server (Netgear WGPS606) but I can print from all PCs.
Since I get the impression that your printer is connected directly to the XP desktop PC, have you set the printer to be shared on the network? Right click the Printer and in Properties, set the sharing option if you haven't already done so.
Before you do anything else, I would unplug the USB for the printer from the XP PC, and just plug it into the laptop. Vista SHOULD install the driver for it automatically (although Status Monitor - ink levels - doesn't seem to have made the Vista switch). At the very least you'll know the printer works under Vista.
If no-one else comes up with anything useful, I'll have a go over the weekend at plugging the printer directly into an XP machine and sharing it on the network to the Vista machine.