The electronic visa has been available to many countries (see list here) for a while now - meaning the online system has been thoroughly tested and applications are very straightforward.
To apply you'll need to head to the government porthole:- https://evisa.xuatnhapcanh.gov.vn/web/guest/khai-thi-thuc-dien-tu/cap-thi-thuc-dien-tu
Then load up a passport photo (this free app makes that simple. Make sure your photo has no shadows and is taken with a white background).
Next upload a clear photo of the ID page of your passport and fill out the form. For your expected address in Vietnam enter your hotel's address, for the entry purpose select du lich (tourism). Once you have completed the form you will be taken through to a payment gateway where you will be charged a flat fee of $25 per visa, once submitted you will be provided with a registration code for your electronic visa - make a note of this, after three days you can see if your application was successful by filling out the form on this page. Just print that out and upon entry to the country take it to the E Visa counter at the airport immigration prior to picking up your bags - in Da Nang this counter is on the left (look for the large E Visa sign).
The visa is valid for 30 days.
For further information visit Vietnam Tourisms official site - http://vietnamtourism.vn/plan-your-trip/official-vietnam-evisa-application