I can't find any restrictions, warnings or maximum safe usage rates on vanillin (and if you have info regarding any of these, please let me know!). The only usage rate info I've got to go on is from the vendor's web site. There it says vanillin can be used up to 8% of the total weight of perfume. I'm assuming that's the undiluted oil blend, not total weight of perfume that is diluted for use.
I looked up the CAS # on different chemical reference sites online, a few perfumery sites, and also in my Essential Oil Safety book by Tisserand/Young. So far, I'm not seeing any max usage restrictions other than food-related restrictions. The 8% suggested usage rate for perfume concentrations is not really what I would consider to be a restriction for safety reasons, but more of a 'quality of scent' concern, based on what I have read so far.
In my Essential Oil Safety book, it was reported that IFRA once had a .5% restriction on it in leave-on products, but they took it off their restricted list pending review of preliminary data presented by "an ECETOC task force that show it to be a weak allergen, only likely to elicit a sensitization reaction if present at 3% or more." (in those who are sensitive to it)
Here is the official announcement of IFRA withdrawing their restrictions on vanillin:
http://www.prnewswire.com/news-releases/ifra-issues-fragrance-standards-update-124583248.html
On this perfumery site (which sells scent components), it makes mention of it being rarely used higher than 8% because it can become 'sickly sweet':
https://pellwall.com/product/vanillin/
Another soaping company that sells vanillin crystals is Dianna's Sundries/Mighty Fine Soap & Candle Works:
https://diannassundries.com/product/vanillin-crystals/
If you look under "Additional Information" on her site, it gives these usage rates:
"Bar Soaps – 2-3 tsp. per pound of oils Lotions – 1-2%."
I'm wondering if I shouldn't see if the crystals will dissolve in water, and if they will just use a little water from my batch to dissolve them rather than using the glycerin mix. I'll need 1-2 tablespoons of the mix PPO at the ratio the vanillin and glycerin are mixed at now, I think, depending on how strong I want the scent. After seeing
BrewerGeorge's results from using glycerin to disperse his colorants, I'm suddenly a little leery of using it at all. I definitely know not to insulate soap made with it!
In the link you supplied, BrewerGeorge said he mixed his colorants into his oil (not glycerin). Did you mean to supply a link to a different thread? For what it's worth, I mix all my colorants in glycerin, soap on the warm side and insulate/force gel, and the glycerin does not cause any issues in my soap. I do soap with a 33% lye concentration, though. Maybe he soaps with a different lye concentration?
HTH!
IrishLass