If I'm remembering Backup correctly, Thomas pulled off a bit of ritual magic to do the tracking spell.
Thaumaturgy kind of ritual, like what Harry does for bigger spells, not "Cosmic Vending Machine"-from-BR kind of ritual. Thomas has genuine magical talent, though less than Harry, and way less training and practice (he compares it to six months at a vocational school vs. Harry having a doctorate).
There's an "unverified" WoJ that:
'Are there White Court vampire wizards?'
Yes, there are. Thomas is middle-of-the-road in power and [ed: think I'm remembering this correctly] the strongest don't get as strong as mortal wizards [/ed], but they can pull off some strong tricks with their Hunger.
http://www.jimbutcheronline.com/bb/index.php/topic,21772.msg947683.html#msg947683My
guess as to why is that the White Court Hunger and their "magic" (feeding/emotion powers/super-strength, speed and healing) interferes somehow with the regular magic. Plus (and maybe more importantly) they tend not to practice as much since they can rely on their other powers, and they may not have other wizards to learn from either.
EDIT:
On vampires and magic, I point first at Vitto Malvora. Then I point at Papa Raith.
Yes, Vitto also used spells, though he had an "external sponsor" (an Outsider). He
probably had spellcasting ability "on his own" but it's not 100% confirmed like Thomas, IMO.
I don't remember Papa Raith ever casting a spell? He had knowledge about the HWWB-ritual, but that's not a self-powered "spell" as such. He had that instant-death "Kiss of Death" thing before Maggie Sr., but that was over-powered WCV powers, not spellcasting. His immunity may be a spell, but could be one cast by Maggie Sr. when they were working together rather than by him -- or it could be an Outsider thing.
