rep-cal is a trusted brand by countless keepers of all types of reptiles, i agree when i think herptivite wouldn't be the problem. however, did you maybe overdose by accident? or over suppliment? and did you mix it 1.1 with calcium or calcium D3? what was the feeder you dusted and was he plenty hydrated while you gave it to him?
yes, there is a wide possibility that he could be allergic to certain vitamin suppliments in the mix, but this would be the case with all multivatamins. rep-cal (imo) balances out a good mix of everything a reptile needs, while some other companies may use all of the same ingredients, but more or less of course.
this is an odd case, so in the mean time, i would research sandras blogs, and stay on top of gutloading as much and as best you can to (in your discression) eliminate vitamin supplimentation all together.
i dont want to say "try the herptivite again", i dont want to give you any harmful advice what so ever, but if the vets cant soon find out what it is, maybe a small test would be the only way to tell. it's your choice and your descression.
if you give him multi-vitamins again and this happens, stop it all together.
i hope this helps and he gets better , cheers