I think many people have made the mistake of voting while being on the Creative server or Map Maker World and therefore not being able to redeem the keys, at least I have, which is why I am making this suggestion.
Virtual keys come with a bunch of advantages over physical keys.
-They can't be duped, so voting on Creative and Map Maker World would no longer mean not being able to redeem keys.
-They are not stored in player inventory. Instead they are stored as a "balance" that is used when opening the crate.
-If implemented correctly, the keys will be spendable on any server, meaning you could vote on the Parkour server and then hop onto the Skyblock server and use your keys there.
This would save new players from not being able to redeem their keys and make life easier for all players.