Yabun ni Kyuuketsu Shitsureishimasu.

Minoru Sayo is an office worker who lives an upright but mundane life. That is, until one day, he turns into a vampire.
Unable to forgo his scruples, he refuses to feed on humans and attempts to soothe his thirst with cat blood. On one such night when he's out prowling for a feline friend, he runs into his coworker in the same department, Yamadera. After a few drinks at a bar, Yamadera falls asleep at the station leaving his vexing neck exposed and vulnerable. Overcome with annoyance and hunger, Sayo takes a bite and discovers...? Will Sayo find a way to become human again? And what really lies behind Yamadera's sweet mask? Take a read and find out in this emotional and twisted tale between a 40-year-old virgin and his sadistic junior colleague!

(Source: MangaPlaza)