A guess for why the "V" is because there are 2 types of vouchers. The 1040-ES and the 1040-V. I had the same question initially because I didn't understand the difference between the two.
But, the 1040-V is for sending in with your current year's tax return when you owe the Feds. The 1040-ES is for paying the following year's taxes. I'm not any kind of expert, so check on the above before you believe it.
