As long as the weapon summoned is a simple, nonmagical weapon, it doesn't violate the VoP.
However, IMHO, an ascetic cannot be forced to drop a weapon that his enemy has enchanted with Magic Weapon or the like. The spirit of the vow is that the ascetic's abilities come from the subject of his vow (a deity or philosophy)- not from the enhancements from his allies. Advantages that come about because of his opponent's attacks would not be covered by this.
However, IMHO, an ascetic cannot be forced to drop a weapon that his enemy has enchanted with Magic Weapon or the like. The spirit of the vow is that the ascetic's abilities come from the subject of his vow (a deity or philosophy)- not from the enhancements from his allies. Advantages that come about because of his opponent's attacks would not be covered by this.