I can only tell you my own experiences...when I used to use Nekton-rep (with preformed vitamin A) I had chameleons with MBD and now I use Rep-Cal's herptivite (beta carotene) and have not had a case of MBD in a chameleon for years. However, I have also changed my methods of gutloading and other things that may be part of the reason for no longer having MBD problems with chameleons.
I have had a lot of chameleons autopsied and only one ever came back with a vitamin A deficiency.
D3 from supplements and preformed vitamin A can build up in the chameleon's system...but lack of both vitamins is also a problem...so its a tough balance to achieve.
This article talks about overdoses and defeciencies...
http://www.seavs.com/case_studies/lizards/chameleons.asp
"# The exact requirements for vitamin A and D are no the same for all chameleon species and are higher for growing juveniles and reproductively active females.
# Excessive vitamin A supplementation may result in interference of vitamin D3 metabolism, leading to MBD.
# Excessive vitamin A supplementation may result in organ toxicity (kidney and liver) some times causing gular edema and reproductive problems such as sterility.
# Inadequate amounts of vitamin A may cause eye problems, neurological dysfunction, spinal kinking, dysecdysis and the formation of hemipenal plugs.
# Excessive vitamin D3 supplementation can result in organ toxicity (gular edema)metastatic calcification, gout, and pseudo-gout (calcium-like deposit).
# Inadequate amounts of vitamin D3 (either in the diet or by conversion from exposure to ultraviolet light) can lead to MBD."
Sue is a well-known vet that has written articles and papers and this is what she has to say...
http://www.rocksolidherpetoculture.com/index.php?option=com_content&task=view&id=57&Itemid=75
"Current thinking is to dust with LOW vitamin A and low-to-medium beta-carotene. Give crickets low-to-medium vitamin A and high beta- carotene."
All I can say is be careful!