A vest fleece is a term first coined by Jack Ciesielski, founder of The Analyst's Accounting Observer, and it relates to stock options that some companies grant to employees. A stock option is when an employer provides employees the opportunity or "option" to purchase shares of the company's stock at a future date at a predetermined set price.
To cash-in on the option or purchase the stock at the set price, the employee must usually remain employed by the company from the time the option is granted until the date that it can be exercised.
At that time the employee is considered vested as defined by the terms of the stock option. A "vest fleece" is when the vesting time, or the date the employee must wait to exercise a particular option, is accelerated. Companies tend to use this strategy when it seems beneficial to how future income will be reported on their profit and loss statements.
For more on this read, ESOs: Introduction.
This question was answered by Katie Adams.