I doubt it is legal, as I never saw any open access information about any vendor. But, maybe somebody can pinpoint me to some clear answer. Thanks!
(I am not a lawyer...)
I can well imagine an unpleasant scenario where someone discloses some technical details of what a particular vendor does, especially if casts the vendor in bad light, or if it discloses some "secret sauce" that most people don't know about (highly unlikely with most vendors) - and the vendor sends lawyer letters citing some non-disclosure agreements that may or may not apply to the poster. That doesn't sound like any fun.
In my opinion, if a question is phrased, for example, "How does
some vendor deal with LIBOR-SOFR migration, possibility of negative interest rates, etc for interest rate options" - that's really not a very good question for this site. Ask the vendor. If the vendor can't give a satisfactory answer, then consider doing business with a different vendor.
Conversely, if the question is "How do various libraries (do the same)?", then it's perfecrly reasonable to say 'some people take the approach of blah blah'. It's very likely that whatever the vendor does is already well-known and can be found in published literature (at least as a pre-print). Cite what's publicly available, even if it's behind a paywall. I've seen tons of proprietary "secret sauce" vendor documents in my life and few of them would ever be worth citing if it were allowed.
If you do know something (pricing methodology or some other useful info) that really isn't published / well-known (this is much more likely to arise at a financial institution rather than at a vendor), and you're concerned about NDAs, then I suggest you search the web for what you're trying to post - if you can cite some paper / book, then do that.