I've definitely seen them at 35 weeks but I don't know that there is a policy--maybe not under that? Depends if it's an easy/low vac or not--I know most of our OBs prefer forceps the earlier the gestation but ideally nothing at all.
check the instructions in the vacuum package &/or the manufacture's website. There may be recommendations there. The vacuum cups (some brands) come in more than one size, so there are probably recommendations for which size to use for which estimated weight.