Vampires being vulnerable to sunlight is a relatively modern concept that dates back to a certain B-grade movie where they tried to hide the fact they had illegally poached the plot of Dracula by changing the way the vampire was killed at the end. As has been pointed out, Dracula did go about in daylight in the book and in the Bram Stoker's Dracula movie.
If some copyright-breaking B-movie director can create a "myth", feel free to uncreate it. Give your vampires whatever restrictions (or lack thereof) you desire.
If it's important to your campaign that most vampires can't go out into sunlight but this one can, just make up any excuse you like.
I've got vampires and werewolves in my cyberpunk game - but they aren't like most portrayals and you can forget daylight, crucifixes, holy water, garlic, wolfsbane, dog roses - they're not going to stop them. OTOH, any bullet can, doesn't have to be silver - but you'd better kill 'em outright because shooting 'em gets them a "tad" annoyed (rhymes with "hissed"...)
It's your game, your vampire, make the rules to suit.
Shouldn't be an issue unless you have a Rule Lawyer in the game, in which case have a bunch of dangerous minions on hand to single him/her out for total annihilation the moment "But the PHB says..." is uttered...
(wibbly wobbly, timey wimey)
