Leadership Vacuum targets the player, not their creatures. A commander with hexproof or protection from blue may be returned this way.

You'll draw a card even if the target player controls no commanders as Leadership Vacuum resolves.

The "commander tax" increases based on how many times a commander was cast from the command zone, not how many times it was put there as a result of Commander rules. The returned commander will cost more to cast again even though it was put directly into the command zone.