You have already demonstrated that you can hook things up in one layer from a connectivity standpoint. This means that you can flood in a FULL Ground Plane on a two layer board and then be able to significantly improve the Power connections on this board and get the bypass closer to the chip pins that need it. Extending Oliver's advice I would suggest that you consider SMT caps for the RS232 level translator chip as well.

I think that this total circuit needs GND plane under it. Not just the USB/FTDI chip parts.

I have used this FTDI chip in several designs with 4 layer boards and had zero problems with them. In one case there were two of the chips used on a board behind a 1->4 USB hub chip inside a piece of lab test equipment that has two 20A SSRs in it perform power switching to server computers under test. We did not experience any USB upset other than the constant issues suffered by USB Comm type devices due to sudden removal of the USB cable while the port is open. The host system software guy for this test setup has made the software smart enough to detect when the sudden removal happens and then invoke system functions that mimic what you can do in Device Manager to forcibly remove hung USB connection so that on next insertion the system can reliably reconnect the same Comm port back up to the running application. Nearly a thousand of these test systems are in use and there has not been any problems with hung FTDI chips.

Michael Karas