Compare Sunbeam's opening line:
"A beam of brilliant light flashes out from your hand in a 5-foot-wide, 60-foot-line."
to Sunburst's opening line:
"Brilliant sunlight flashes in a 60-foot radius centered on a point you choose within range."
This seems to suggest that, for the purposes of Sunlight Hypersensitivity (e.g., that of Vampires), Sunbeam and Sunburst are different. However, Sunbeam has a second component:
"For the duration, a mote of brilliant radiance shines in your hand. It sheds bright light in a 30-foot radius and dim light for an additional 30 feet. The light is sunlight."
Does the sunlight "property" apply only to the mote, or to the beam attack as well? RAW, it would seem like the former (based on the quotes above; the name of the spell suggests otherwise), but it's counterintuitive that the least powerful component of a spell (non-damage dealing, presumably 'utility-oriented') has a damage-dealing property that the more powerful, damage-dealing one doesn't.
One other wrinkle:
RAW, it would seem that Sunburst doesn't trigger SHS, despite the fact that it's a L8 spell that explicitly creates sunlight. Is it reasonable to house-rule that the affected vampire(s) takes the 20 damage, at the time of casting (even if they succeed the saving throw, since the entire burst is still sunlight)?