You can hold it to reduce taxes right?
If you then hold and it goes above $100 while you held, you pay capital gains tax (short or long-term, depending on how long you hold) when you sell.
Again, exactly the same as would happen if you bought those shares yourself with a cash bonus. Getting shares really is the same as cash, for tax purposes.
If you sell on vest, you pay income tax and 0 capital gains. If you sell after vest, you pay the same amount of income tax, and possibly nonzero capital gains.
At least, that's my understanding. I'm not a tax accountant, though. This is not financial advice.