Thank you for the clarification! I had just ordered my first batch of LDRs for messing around just before I saw this post. It's good to know that if results aren't the greatest, there is an upgrade.
Happy to help. I'd love to know if there are any LDRs that are really quite quick and responsive to a good LED, as compared to the results I get with the VTL5C3. My knowledge/experience doesn't go far here, though I have a small pile of LDRs still in the packaging, from back when I was determined to find an LDR solution that was ideal for circuits (lightning fast response with linear, high-dark resistance is my ideal in any circuit IMO). If someone else knows of such an LDR, and can blast a hole through my verbose generality of LDRs being at least slow compared to fastest available photocouplers, I'd love to know so I could order some of them myself!
Plenty of photocouplers are slow to snail-like too, by the way! The VTL5C2 I have is a sloth. I'd just as soon use an average LDR/LED. Maybe it's damaged, but the specs on the datasheet don't tout speed from what I can read.
In many write-ups about circuits, people just indicate using and LED+LDR or photocoupler as if 'any will do.' I first tried optical junk with a tremulus lune and thankfully ordered/used a VTL5C3, had no issues, and thought nothing of it. My bias is that I am as rather avoidant of parts mojo and annoyed by circuits where component choice is critical. TAOE reinforced this as a bad circuit design practice anyhow, but hey, this is audio electronics.
Come to find from subjective expirimentation, a circuit that drives an optical component with am effect that responds to an optical component just invites component specification detail if not an exact production component with tight tolerances. Go figure. As a disclaimer to all I write though, I'm just a hobbyist hack in comparison to many around here, don't be mistaken...
Anyhow, as for current production optical compoents, the Vactec stuff gets chatted up the most around the pedal and audio circuitry realm - that's what I can see. Prices correspond generally speaking. However, from the datasheets of all the vactec and silonex parts I have come across (and from the parts I tell myself I can comprehend), the Silonex NSL-32SR3 which smallbear also sells, does seems to indicate quick responsive specs that would yield a big squash in the flatline compressor like the VTL5C3. So if you want something close to what the VTL5C3 does, but only 1/2 the cost as far as smallbear's pricing currently goes, it may be worth the risk. I have no personal experience with that part though, so I can't really offer a subjective comparison beyond my personal limitations in knowledge which I detail above. I do hope to have one or two in hand at some point to try in a flatline to be more definitive, even if I abandon that circuit for my current purposes. If those characteristics don't interest you for any such reason, you may be quite happy or even happier with what you find on hand than you would with the photocouplers I'm chatting up here, and I hope I am in the minority, as going back to your original post, the LDR/LED method is certainly cheaper across the board....