Data rate is low. 50 bytes in 2 seconds. 2400 serial port baud.
TOTALLY IRRELEVANT. the thing that makes layout critical is NOT frequency, but rise and fall times.

I have used this FTDI chip in several designs with 4 layer boards and had zero problems with them.

the trouble with schematics is that they connect a to b and many assuumes a trace on a PCB will do the same. this is not the case. A trace is not a DIRECT connection, it is a mixture of inductors, capacitors and (OK Per) resistors.

when gnd and Vcc are run through a trace every chip 'sees' what any other chip is doing.

when you throw a 5ns risetime into such a trace all strange thing start to happen.

to minimise these effects I recommend always to use a 4 layer board with solid ground and Vcc planes.

in a pinch, you may get by with a 2 layer board with a solid ground plane if EVERYTHING is decoupled carefully and fully.

Erik