A century ago, the Vampire King Duzell was defeated by King Phelios, who also died in the battle. However, Duzell placed a curse on Phelios, and promised that his reincarnation would destroy the reincarnation of his enemy in the future. Now, Duzell has been reborn in the body of a feline kyawl and finds himself in the company of Lady Ishtar, a descendant of King Phelios. Together, the two decide to find Duzell’s long time foe and set forth on a grand adventure of love, tournaments, and laughter.
Primula is a princess of the small kingdom of Beazol. As a young child, she was entrusted to the care of a boy named Argent. But unbeknownst to Primula, her new friend was actually a Basilisk - a slave trained by an ancient organization to be a sold as a food taster or assassin. Due to their constant exposure to poison, a Basilisk's hair turns white and they are unable to physically touch another, lest they inflict death upon them. After a misunderstanding, Argent was cast out of the castle and Primula lost her dear friend; but after being poisoned with mercury in the present, Argent is once more brought into the princess's life. Though he still harbors bad feelings for Argent, the King hires the man to watch over his daughter and the two begin their new life together. Political troubles, Primula's duty to be married and a number of other obstacles stand in their way, not to mention the fact that the two seem to be falling in love. Can the two ever be together when Argent's very touch will kill the princess?
Vampire Game and Apothecarius Argentum definitely gave me the same vibe. Both take place in somewhat medievel times with fantasy elements, have plenty of bishies and a somewhat political (or at least, somewhat aristocratic) plot. Each involves an unlikely sort of hero and a princess who wants desperately to be her own person. I dunno, in general these two just felt very similar to me.
Both series are shoujo manga in which the main male character is a supernatural being (a vampire in Vampire Game and a demon in Mugen Spiral) who gets trapped in (or reincarnated in the case of Vampire Game) a form of a cat/cat-like creature. Also, both manga have a strong and/or fiesty female lead character. Despite having different plots, both series share a similar feel to each other. While neither series is technically a shounen-ai series (after all, they both have a main female character), they both contain mild shounen-ai hints/senarios/sub-plots (more so in Vampire Game). It should be noted that Vampire Game is a much longer series than Mugen Spiral. Still, if you like one, you will probably like the other.