Perhaps fighting alone for years is finally taking its toll and he had to take a break. Or he was being nice and decided to share screen time with his buddies.
Nah, this was neither of those. Luck just likes to watch, he's using his buddies as bait, to see if the vampires will use any fancy magic he doesn't already know.
Vampires are very prideful people. They refuse to recognize that Majin are stronger and better-organized than they are. They believe in the power of their own scheming. And with 1000 evil dragons, they have a chance. And that true ancestor sounds strong too.
Just one dragon would not be enough, though. It will just be used to show off how much stronger the MCs have become since the flashback.
I think the only one who did it right, is the protagonist of "Demon swordsman of a holy sword academy". Who turned into a cute shota and infiltrated a military academy of one of the strongest powers of the world after thousands of years have passed.
Because if you've been sealed for a long time, your information is probably thousands of years out of date. And that is a lot of time even in a fantasy world.