Final answer:
The main difference is that the perfect competitor's VMPL equates the additional revenue of one more unit of labor with market price, while the imperfect competitor's MRPL considers the need to lower prices to sell additional output, making MR less than price.
Step-by-step explanation:
The difference between the perfect competitor’s VMPL curve and the imperfect competitor’s MRPL curve concerns how each type of firm perceives and responds to the market when they sell additional units of output and employ additional units of labor.
For a perfect competitor, the Value of the Marginal Product of Labour (VMPL) is the additional revenue a firm earns by employing one more unit of labor, holding all else constant. Since a perfect competitor faces a horizontal demand curve, they do not have to lower the price to sell additional output, hence the VMPL is equal to the product of the marginal product of labor (MPL) and the market price (P).
In contrast, an imperfect competitor, which faces a downward-sloping demand curve, must reduce the price of their product to sell additional units. As a result, the Marginal Revenue Product of Labour (MRPL) reflects the additional revenue from employing one more unit of labor, considering that the marginal revenue (MR) is less than the price due to the necessity to lower the price to sell more output. Hence, the MRPL takes into account that the sale of additional output requires a cut in price.