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This is actually a somewhat complicated question but here's what I've worked out.
A vampires sunlight vulnerability is a Weakness, and while weaknesses can be of any type (SU, EX, ect) a vampires weaknesses are not listed as any given type. However most sunlight related weaknesses are Ex, though without it being specifically stated we can't say for certain.
Change Shape works like a Polymorph spell, which makes no direct mention of losing past Weaknesses or qualities, some are gained and a few are changed but none seem to be directly removed unless the new shape would obviously not have them (changing from a lion to a lamb makes you lose your claws, for example). The spells that Polymorph mimics (Elemental Body, Beast Shape and Alter Self) also do not state a specific loss of Weaknesses, so I would rule that they stay when a vampire assumes any of their other forms. It could be debated otherwise but as per the RAW as I understand it the weaknesses most likely stay.
Gaseous Form is slightly different but follows the same basic rules. While the subject does lose it's supernatural abilities it does not seem to loss any weaknesses specifically. In this case it depends on what you rule a vampires weaknesses to be, supernatural or extraordinary. If they're supernatural than the vampire can survive as gas in sunlight, if it's extraordinary than it cannot.
On a personal level I would rule that a vampire dies in sunlight no matter what its form is, only by travelling in a coffin or other sunlight proof means of travel would it be possible. Phew :P
Unlike 3.5 the polymorph school, Pathfinder does not change your creature type so you are still vulnerable to sunlight and other weaknesses.