Since it's an instant, you simply pass your turn, wait until your opponent's upkeep, and then cast it, which is very close to getting an extra turn (the main difference is that your opponent will get to untap)
By the way it doesnt matter if Fireball revolves or not
There are plenty of Equipment cards that grant the equipped creature deathtouch: Basilisk Collar, Gorgon Flail, Gorgons Head, Quietus Spike, Reapers Talisman, Vorpal Sword, and Wand of Orcus
There are very few other cards in Magic that can retool cards in such a warping manner, providing vampires a flavorful and unique trick up their sleeves
The last point is the most important to me, as there is no shortage of deck experience in these veteran bones