There are two scenarios:
The "token_unit" is "$from.vesting" - This means that if you contribute STEEM to the ICO you get VESTED SMTs directly when the ICO launches.
The "steem_unit" is "$from.vesting" - This means that you can participate in the ICO by simply showing that you have vested STEEM. I guess you could call this a "proof of STEEM stake" ICO?
I agree with (1), but think that it needs to be new vesting rather than proof of existing vesting.
Yes, on re-read you are correct. The whitepaper says:
So it is newly vested STEEM (to yourself) which is your payment into the ICO. Not sure why anyone would really want to do this, but it's an interesting concept. Would love to hear a use case.