Is it permissible to offer a vow, which stipulated that it be food for the poor, to poor parents?

Undertaking a conditional vow (nadhr mu'allaq) is disliked (makrooh). The vow must be fulfilled as intended, by purchasing food and distributing it to the poor. If the maintenance of your parents is obligatory upon you due to their poverty, then it is not permissible to give the vow's fulfillment to them. It is also not permissible to substitute the vow except with something better than it.