Yes Noga is good ref and I have a few others as well, i.e., Kingsford, Stoskopf, Herwig, Couch, Clifton and Bassleer.
"no effect on tomont division, but it kills dinospores immediately on their encystment."
Tomont = after if falls off the the fish and settles to the bottom where is then goes through cellular division and then releases water born Dinospores. On attachment to the fish the Dino's turn into feeding Trophonts. It is at this first attachment where the Dinspore are killed directly from the drug. I see little info on actually killing the parasite once it has reached full Trophont stage on the fish. But I would think so as it would be adsorbing the drug from the fish here also.
Question
What makes you thing you have Velvet ? Did you ID it as a Dinoflagellate ? Have you mis-ID by observation and you really have Brooklynella hostillis. Are you sure it is not just "ick" and not a clouding of the skin caused by excessive production of protective mucus by the fish misleading to a ID as Velvet ?
One of the theories on Velvet is to get the fish to over mucate in the presence of Copper @ 0.15 ppm. It is believed it is the ability of the mucus to concentrate high levels of Copper. In this case copper does not kill the parasite either but breaks its cycle.
Research done by Bower years ago showed that under the right conditions, which have not been really ID, at times the Dino's can still be found active for almost 40 day but these are in untreated tanks. Even bleach at 3 tsp / 10gal for 24 hrs. does not phase the Tomonts, yet there is 100 % death of all bacteria. FW also does about Zippo for Tomonts and are fine after 6 wks in FW but do better in tap water than distilled water.