Welcome to the forum!
Most vitamins are water soluble and you shouldn’t be able to overdose them since they should be excreted…but that doesn’t mean go crazy with them. The ones to be careful with are the fat soluble ones like prEformed vitamin A and vitamin D3 because they can build up in the chameleons’ system and lead to overdose. These two you only want to provide lightly twice a month.
D3, from the proper UVB lights we recommend using (Reptisun 5.0 or Arcadia 6%, long linear tube UvB lights) and from direct sunlight (that does not pass through glass or plastic), should not build up as long as the chameleons can move in and out of it at will. D3 from these sources is produced in the body as needed. PrEformed vitamin A is ready to go and will also build up in the chameleon’s system. PrOformed sources of vitamin A won’t build up, but it’s not known to be converted into prEformed by the chameleon…or not converted efficiently enough, so are considered not necessary/useful to the chameleon.
We recommend to dust the insects, just before feeding them to the chameleon, lightly at all feedings but 2a month with a phos free calcium powder and then on the two other feedings, once every two weeks, dust lightly with a vitamin powder that contains a prEformed source of vitamin A and D3 on those two days.
Hope this helps a bit!