Earlier quoted context omitted.
Vested is not exercised. Options vest, but you have to exercise them to purchase the underlying shares. This is nominally cheap, but from the IRS’ perspective you have just spent $1 to purchase a share worth $100, so that’s $99 of income. Multiply by a large number of options and you can easily have a real multimillion dollar tax bill even though you have no way to sell the shares to recoup their value. Worse, if the…
Doesn’t income happen when you sell the shares ? What is the cost basis of the shares you purchase if not the strike price of the option ?
At the same time, if it's an ISO option, there is no assessment of ordinary tax until the stock is disposed. If there's a merger and the proper forms are followed, you can be issued stock from the acquirer that retains the basis (the strike price) of the original shares. If the forms are not followed, the acquisition is a taxable disposition.
There's a credit for the difference between AMT tax and ordinary tax on ISOs, but it can take many years for that to fully work out, and you have to have paid the AMT in the meantime.
Early exercise with 83(b) at time of grant (or while the value hasn't changed) or exercise-and-sell make the taxes make the taxes simplest, but tax simplicity isn't always the best strategy.