You are a vampire. You are moving in on these interloping fools with their lanterns and torches. Their weapons are ineffective. Their [justified] fear has their tactics all muddled. You've charmed their lead warrior who actually has some ancient weapon that appears capable of wounding you. She will make an acceptable addition to your guardians...for a time. Now to remove the quivering man toting the symbols of the southerners' storm goddess before he can try to call on the divine power that can harm you.
*FWOOM!!!!*
The chamber immediately becomes bright as the noon day sun. The sudden illumination forces you to squint. With a shriek and single bound, you remove yourself out of the area of hateful brightness that might be your final doom. You are able to tell, as you melt back into the shadows to remove yourself to your great chamber/ledge/known hiding/ambush place, it seems an elfin sorcerer in the rear is the source. You silently vow him an especially gruesome end when next you meet. "DEFEND ME!" you shout and the great warrior-woman you charmed raises her shield and retreats with you while her compatriots shout out her name.
As a powerful immortal undead....are you sitting around waiting to see if you start to smoke and scorch when your catacomb looks like a sun-bathed field?
Or are you getting the hell out of there BEFORE you start to turn to dust?
The vampire is not sitting there saying "Hmm. I don't SEEM to be taking 20 radiant damage. There's no "lasers" shooting out of the sorcerer's hand. Maybe its not that extremely rare and powerful spell that can destroy me (assuming you have some way of having in-world knowledge of the sunbeam spell in the first place)." All YOU know is that, some magic has caused "DAYLIGHT" to happen where you are.
As always, different strokes and play what you like. But I'm not playing intelligent uber-powerful undead creatures, with a single (other than running water) weakness they must avoid at all costs, as dullards gambling around with their immortality, "Let me sit around and see if I begin to disintegrate from this 'bright [DAY] light' appears around me."
tl:dr/IOW: Daylight doesn't have to produce actual "sun light" or have the same description as Sunbeam to effect a vampire. You don't need the spell to define the same effect to have an effect that is the same.
And PS: AFAIAC Sunbeam is justified as a 6th level spell because you can actual aim/choose targets and shoot laser beams along with/in addition to shedding bright light in whatever radius and dim light beyond it. Not because it is "sun light" because the description says so, but 3rd level Daylight isn't because the description doesn't use the word "sun." This is the very definition of pedantry.