The most recent version of "AltiVec Technology Programming Instructions Manual for Power ISA Processors" (ALTIVECPOWERISAPIM.pdf) is preliminary and not published yet.
If you wish to get preliminary version of this document, please contact your local Freescale representative.
Looking to this document, I see the vec_malloc() is somehow mentioned in Section 3.5, but there is no changes in this section, this section is exactly the same as in old ALTIVECPIM.pdf (6/1999).
If the function have no prototype this means you have some library missing.