I have been lookig at some other aspects of this project. One thing that had bugged me for a while is that the opto LED only works on one half cycle of the audio waveform. You sometimes get asymmetrical waveforms so with only looking at one of them you could get over or under compression. Most pro compressors use full wave rectification so both halves af the waveform are taken into account so I have devised a circuit to do this:

The four 1N4148 diodes form the required bridge and I have moved R4 and the gain reduction meter to the other side of it so they are ground referenced. If have done some simulations of this circuit to verify that it should work but I have not built it. The diodes do raise the threshold by two diode drops but there's enough output swing in the TLA to overcome that. The sims indicate that, with R4 set to 1K, just under +9dBV at the output is required for 1mA LED current and +20dBV is required for 10mA LED current.
Lastly a quick word about Vactrol types and R1, R2. In this compressor,as in all LA2A lookalikes, attack and decay time, program level dependent performance and the ratio are entirely dictated by the chosen Vactrol. These factors also vary with drive level and with the length of time they are operated. Furthermore, the attack and decay times given in the datasheets only apply at a minimum current of 10mA which is probably the maximum current they will ever see in this design. So the spec sheets are at best only a guide to the figures you will actually obtain. Having said that, Vactrols tends to fall into broad categories so we can make some generalisations about the most common ones.
VTL5C1. attack 2.5mS, decay 35mS. Resistance, approx 20K at 1mA and 600R at 10mA. Very wide dynamic range. Should make a relatively fast compressor. Try R1 = 100K
VTL5C2. attack 3.5mS, decay 500mS. Resistance, approx 5K5 at 1mA and 800R at 10mA. Fair dynamic range. Slow decay so probably better as a limiter. Try R1 = 68K
VTL5C3. attack 2.5mS, decay 35mS. Resistance, approx 30K at 1mA and 5R at 10mA. Good dynamic range. I have used these and they do make a relatively fast compressor. Try R1 = 100K. A 20dB gain reduction is possible.
VTL5C4. attack 6mS, decay 1.5secs. Resistance, approx 1K2 at 1mA and 125R at 10mA. Fair dynamic range. Very slow decay so probably better as a limiter. Try R1 = 22K
Bear in mind that in all cases the threshold is determined by the forward voltage of the opto LED and any other diodes in series with it. So, in the simple version you need about 1.5V peak or 1.1V rms or about +2.7dBu at the LED before any gain reduction occurs. If you set the side chain TLA gain to 30dB then your lowest threshold will be about -27dBu. If you use the bridge, the threshold at the LED rises to about 2.8V peak or 2V rms or about 8dBu so the lowest threshold rises to -22dBu.
Cheers
Ian