It’s not necessarily. Incase you have booked the flight ticket to Vietnam so you can put the exact date of arrival otherwise it should be the earlier than your plan. For example, you applied for the 3-months visa with arrival date as Dec 5th, 2018, the visa letter will be valid until Mar 5th, 2019. You can arrive anytime between this period but should leave the country by Mar 5th, 2019 or apply to do visa extension before it is expired.
Note: The Vietnam visa approval letter could not be amended once it is issued, so you have to apply for a new visa if you want to visit Vietnam earlier or change it to another type of visa.