As usual feel free to take those ideas and run with them.
See the thing about sending Megumi is that it doesn't preclude her brother or the sisters from showing up. With mirrors and hot water as their teleport medium its not an issue... you can even get away with FoZ being on another planet in the same galaxy. Then it becomes a matter of paperwork. With K1 you run into angry Belldandy and a possible interdimensional divine incident. With Megumi its her being annoyed and her life disrupted... how that is handled is a lot of the story. You can also do weird things like having Megumi get dosed with the love potion and barely notice from a combination of magic resistance (she kept getting hit with just a dream counters to incidents and only gets exposed to divine/infernal level magic so much it can be her gaining unreliable spell resistant to be abused for cracktasicness or drama.) That and/or have the Water Spirit decide to switch out the potion effect for something amusingly annoying until she gets it back its ring. She is really amusingly prone to being annoyed by the supernatural... but it never really lasts long enough to do too much damage.
The problem with Vash is that you'd need someone that matched up with him enough to qualify as his match... and FoZ is too full of @^*%&^ people to qualify... unless you used that half-elf void mage it wouldn't really work... and its likely sacrilege to let her use the ceremony... which would make her wanted in the worst way... which would make her more Vash like. Which means the main issue is Vash would rapidly run out off bullets.
See the thing about sending Megumi is that it doesn't preclude her brother or the sisters from showing up. With mirrors and hot water as their teleport medium its not an issue... you can even get away with FoZ being on another planet in the same galaxy. Then it becomes a matter of paperwork. With K1 you run into angry Belldandy and a possible interdimensional divine incident. With Megumi its her being annoyed and her life disrupted... how that is handled is a lot of the story. You can also do weird things like having Megumi get dosed with the love potion and barely notice from a combination of magic resistance (she kept getting hit with just a dream counters to incidents and only gets exposed to divine/infernal level magic so much it can be her gaining unreliable spell resistant to be abused for cracktasicness or drama.) That and/or have the Water Spirit decide to switch out the potion effect for something amusingly annoying until she gets it back its ring. She is really amusingly prone to being annoyed by the supernatural... but it never really lasts long enough to do too much damage.
The problem with Vash is that you'd need someone that matched up with him enough to qualify as his match... and FoZ is too full of @^*%&^ people to qualify... unless you used that half-elf void mage it wouldn't really work... and its likely sacrilege to let her use the ceremony... which would make her wanted in the worst way... which would make her more Vash like. Which means the main issue is Vash would rapidly run out off bullets.