How to find "Market Price on Vesting Date (Per Share)"?

You could use E-Trade stock look-up or research tools to determine the market price for the security on the day the shares vested with you.  Or you could use the method you describe which is to divide the sale proceeds by the number of shares sold.  However, if there were any commissions/fees charged on the sale, you will want to add those commissions/fees back in to the sales proceeds to determine the market price (assuming the sale proceeds were net of commissions/fees).  However, if you need to determine your cost basis for the shares in question, then include the commission/fees in your cost basis.  

Alternatively, you could use a third party website such as Yahoo Finance to find the market price for the vesting day.  You might want to use your method of dividing the proceeds by the shares sold and another method such as Yahoo Finance to confirm the market price.  

@gsu