Found the answer to my own question. Apparently, you can pass by reference, but the function MUST be cdef
'ed, not def
'ed. i.e.
# somefile.pyx
#distutils: language = c++
from libcpp.vector cimport vector
cdef void add_one(vector[int]& vect):
cdef int i
n = vect.size()
for i in range(<int>n):
vect[i] += 1
cdef vector[int] v
for i in range(100000):
v.push_back(i)
add_one(v)