Indeed if a Sunburst spell goes off in the vicinity of a vampire it is going to be painful even if the vampire has Protective Penumbra spell running.
The way I see it and understand it:
The vampire will take 50% additional damage (if facing a 20th level caster 20d6 plus 50% of the value of that roll). It's not an energy type but the vampire is protected from penalties caused by the vulnerability so no -4 on the Reflex save. Whether you are destroyed if you fail the save depends on your GM (and/or if there is a FAQ or clarification out there). A more literal interpretation says you won't suffer the -4 penalty to the Reflex save but would be destroyed based off this part of the text -> "Some creatures might suffer additional effects, as noted in their descriptions." Being destroyed is not a penalty as it is not a 'numerical value' -> Penalty: Penalties are numerical values that are subtracted from a check or statistical score. Check with your GM as your unlife literally might depend on it. In any case the Protective Penumbra gets dispelled.
Of course, as Cyrus eludes to, if you are standing outside during daylight when the Sunburst hits you are in deep doo anyway (have your Contingency cast). Of course one way to make this less an issue is to Heighten your Protective Penumbra to 9th (that will cancel out an awful lot of light spells with text along the lines similar to that of Sunburst, 'dispels any darkness spells of lower than 9th level within its area'. A friendly divine caster with access to Greater Spell Immunity can also be of great value in protecting you from the few highly dangerous 'light' spells out there (or a high UMD and the right item/scroll). There are 3 I'm aware of and would take care to protect myself from are: Sunburst (8th Wiz, Drd), Sunbeam(7th Drd) and Searing Light(3rd Clr). Both Sunbeam and Sunburst are capable of outright destroying undead with weakness to sunlight.