Would receiving a crafted contingent spell break a vow of poverty from the Book of Exalted Deeds? For example, if the players are tasked with helping a monastery in the mountains repel some sort of calamity, and all the monks and clerics have taken vows of poverty/celibacy/etc, would the player's wizard be able to craft useful contingent spells on some of the npcs or would that break their vow? It boils down to whether or not those contingent spells count as possessions or not.
Yes, he'd break it
The Craft Contingent Spell [Item Creation] (emphasis will be important later) feat (Complete Arcane p.77) says:
...Crafting a contingent spell takes one day for each 1,000 gp in its base price (spell level × caster level × 100 gp). To craft a contingent spell, you must spend 1/25 of this base price in XP and use up raw materials costing one-half the base price. Some spells incur extra costs in material components or XP (as noted in their descriptions), which must be paid when the contingent spell is created...
Vow of Poverty (Book of Exalted deeds p.48) says (with emphasis added):
...you must not own or use any material possessions... You may not use any magic item of any sort, though you can benefit from magic items used on your behalf... You may not, however, "borrow" a cloak of resistance or any other magic item from a companion for even a single round, nor may you yourself cast a spell from a scroll, wand, or staff. If you break your vow, you immediately and irrevocably lose the benefit of this feat. You may not take another feat to replace it.
The contingent spell is a magic item crafted like most any other item, just with an odd trigger. Any character with a Vow of Poverty may not use any magic item of any sort. He could still benefit from a contingent spell on another character that just happens to include or target him once it's been activated, however.
I like Chemus' answer, but it still does not support definitively that contingent spells are in fact items. I did a little more digging and found this section from Complete Arcane, page 139, under "New Types of Items:"
The basic types of magic items described in the Dungeon Master’s Guide—armor, weapons, potions, scrolls, rods, rings, staffs, wands, and wondrous items—are not necessarily the only types of magic item possible. In Faerûn, the world of the FORGOTTEN REALMS campaign setting, magic tattoos, magic runes, and contingent spells are common magic items, each crafted in the same manner as a standard magic item and requiring its own unique item creation feat (Tattoo Magic, Inscribe Rune, and Craft Contingent Spell).
This defines explicitly that contingent spells are definitively items, which would make them break vow of poverty. I feel this section was necessary to find because contingent spells otherwise can be dispelled by dispel magic, which is contrary to the function of other magic items, plus the feat itself only ever refers to them as spells and not items.
If you have to ask, it probably does
While the DM is the ultimate authority on what unmentioned items and effects do or don't revoke the benefits of a creature's Vow of Poverty feat (Book of Exalted Deeds 48), some additional guidelines as to what may break a creature's Vow of Poverty are provided by the Sage in the Dragon #324 and #343 Sage Advice columns (and repeated in the FAQ) and in the D&D Q&A Web column "10/18/2007." This Sage advice can be summarized by this sentiment: "The Sage strongly suggests that if you’re trying to find ways around the various Vow feats in [the Book of Exalted Deed], you may be missing the point of the feats." In other words, if you have to ask if something violates the Vow of Poverty, then whatever it is probably does violate the Vow of Poverty.
For example, in the Dragon #343 Sage Advice column "Official Answers to Your Questions," the Sage is asked if the Vow of Poverty's strictures are violated by magical grafts—magic items that often replace or add body parts and that can be added to a creature without the creature's permission (as could happen were the creature experimented on by a renegade silthilar (Lords of Madness 168-70)). The Sage replies, in part, that grafts should violate the Vow of Poverty and that "physical removal of the graft—which might well require amputation or the like—is almost certainly necessary" to maintain the Vow.
So if a creature must lop off its own involuntarily acquired replacement hand to maintain its Vow of Poverty, voluntarily receiving a contingent spell also likely violates the Vow of Poverty.
In other words, this DM would rule that a contingent spell—despite not being a physical item—remains a thing of value that can be disposed of and to keep the benefits of the feat Vow of Poverty the creature must dispose of it. Thus this DM would say that the creature violated its Vow of Poverty by acquiring the contingent spell voluntarily, and the creature should seek the contingent spell's removal immediately upon its discovery if its acquisition were accidental or inadvertent (like if a contingent spell's creator straight-up lies to an ignorant monk about the effects of the 10-day ritual he's performing on him).
(As an aside, in this DM's opinion one of the few others things—besides what's listed in the Vow of Poverty itself—that does not violate the Vow yet that still possesses a gp value is the effects of magical locations—like the infamous Otyugh Hole (Complete Scoundrel 152). These sites are given a gp value that's subtracted from a dungeon's treasure by the DM. Even on these, though, another DM may rule differently.)
Note: None of this has anything to do with the relative power of the feat Vow of Poverty (which is viewed as too low by far by this DM and player), but, instead, with the feat Vow of Poverty as a mechanical and role-playing exercise. As always, this DM strongly recommends against any character taking the feat Vow of Poverty. Instead, this DM recommends a player wanting to play a PC with a vow of poverty (n.b. not the feat) simply give away all his treasure and seek alternative rewards. This, in this DM's and player's opinion, is better than, for example, the inevitable fights that erupt over whether a creature violates the feat Vow of Poverty by, for example, unknowingly inheriting an estate in a foreign land.