I have called the Wyeth hotline yesterday. The lady confirmed that there was no additive in Stage 1 powder (it is not allowed by the Gov't) but "vanila favour" (乙基香草醛) was added in Stage 2 and Stage 3 powder. She stressed that the amount was within the limit governed by the local authority and was safe.
It seems that it is quite common for manufacturers to add some favour in Stage 2 formula to attract older babies. Even Illuma has such ingredient. Hope it really has no side effect.