A pilot e-visa program launched in 2017 allows visitors from 40 eligible countries to apply for visas online through the Vietnam Immigration Department. E-visas are single-entry, valid for 30 days, and cost US $25. While you apply online, and can print visa, you still have to present it for validation when you enter Vietnam, actually serving as a Visa on Arrival.
Once in the country, one option is to request an extension at the Vietnam Immigration Office; the Hanoi location is 44 Trần Phú, Điện Biên. Another is at 161 Nguyen Du Street, Ho Chi Minh City.
Vietnam does authorize visa agencies, and you may wish to use such services to extend your stay. A quick search (no affiliation) returns Vietnam-eVisa, Vietnam Immigration, and Vietnam Visa Easy as some options.
As with most in-country visa extension processes, it should be done prior the expiration of your current visa, and should be requested, at a minimum, 4-7 days in advance.