"Mysterious Triangle" targets the monster it destroys. If "Vanity’s Fiend" is preventing Special Summons, and it has an A-Counter, you can destroy it with "Mysterious Triangle" then Special Summon an Alien. If an effect is chained to "Mysterious Triangle" that removes the target’s A-Counter, it is not destroyed. (Example: if it is flipped face-down with "Book of Moon".) If the targeted monster is not destroyed for some reason, no Alien is Special Summoned.
If an effect is chained to "Mysterious Triangle" that removes the target's A-Counter, it is not destroyed. (Example: if it is flipped face-down with "Book of Moon".)
If the targeted monster is not destroyed for some reason, no Alien is Special Summoned.
"Mysterious Triangle" targets the monster it destroys.
If "Vanity's Fiend" is preventing Special Summons, and it has an A-Counter, you can destroy it with "Mysterious Triangle" then Special Summon an Alien.