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.

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.