What is the interaction between a vampire with the Endure Sunlight feat (Libris Mortis) and the sunburst (and sunbeam, etc.) spell?
Does Endure Sunlight provide any defence against the potential instant death from those spells?
The description for Endure Sunlight states:
You can resist all dangerous effects of sunlight for a number of rounds equal to 1 + your Charisma modifier (minimum 1 round). After this time, if you are still exposed to sunlight, you take the normal effects as appropriate for your kind.
While the description for Sunburst states:
An undead creature caught within the globe takes 1d6 points of damage per caster level (maximum 25d6), or half damage if a Reflex save is successful. In addition, the burst results in the destruction of any undead creature specifically harmed by bright light if it fails its save.
Due to the fact that the Sunburst spell specifically designates "Undead creatures" and not "Creatures vulnerable to light", Endure Sunlight is not a protector for the instant death effects of Sunburst. It also wouldn't protect against the blindness of the spell effect, because the blindness takes place no matter your type or vulnerability.
However... if you're looking for protection from sunlight without burning a feat, a cantrip exists cross-system in Pathfinder that you may be interested in, that you could adapt to a vampire character in a 3.5 game pending GM permission. However, the cantrip is a Rare cantrip, so you would need to do research to discover the cantrip to learn it or add it to your spellbook / spell list:
(Pathfinder Rare Cantrip)
School evocation [darkness]; Level sorcerer/wizard 0
Casting Time 1 standard action
Components V, S, M (a bit of soot)
Target creature or object touched
Duration 10 minutes/level (D)
Saving Throw Will negates (harmless); Spell Resistance yes
This spell keeps the creature or object touched slightly in shadow. The target of this spell does not suffer any penalties or blindness caused by bright light, such as those from light sensitivity or light blindness.
You can have only one penumbra spell active at any one time. If you cast this spell while another casting is still in effect, the previous casting is dispelled.