Neither of those are possible. The worst case scenario is a 15% return on your money, even if the stock does nothing but tank over the entire period. You purchase the stock at a 15% discount at the lower of the price at either the beginning or end of the period.
If the stock was at $100 at the beginning and $120 at the end and you put in $1000 you would get 11.7 shares of VMW stock (purchased at $85 each) with a current market value of $1411, you can sell that on the open market the next day for a 41% return.
Likewise if you put in $1000 and the stock went from $100 to $80 over the period you would get 14.7 shares (purchased at $68) with a market value of $1176, a 17% return. It doesn't matter how much the stock goes down, you're still buying a thousand dollars worth of stock at 15% less than market value, even if the stock went from $100 to $10 you'd buy 117.6 shares for $8.5 (which would be worth $1176 at $10 a share).