Here's my experience with this Mod.
As you probably figured out, I bought a 5 legs VTL5C4/2 so I can do the Mod, it works BUT.... It has some influence on the audio path!!
There's more gain when the led is connected, and with a fairly hot output guitar (Seymour Duncan SH4) some distortion can be heard when played hard. This is the same problem as described here: http://www.buildyourownclone.com/board/viewtopic.php?f=19&t=25117
and I've tried 9 and 12V power.
My solution was adding a resistor between the optocoupler and the LED, and, it may seems silly, be carefull to choose where to solder the LED to the ground: soldering it on the output jack made things worse, on the pcb it sounds better. For the resistor I haven't a lot of values to try but 4.7K seems fine, almost no distortion and enough juice to light up the LED.
Off Topic: I've also tried a different IC, the OPA2604 and was disapointed: the sound is indeed clear but at the expense of the compression as the sensivity pot has no effect, as the attack switch.
thank you for reading!