Three ways to stay in Vietnam longer. Each one works differently.
You leave Vietnam by land and come back on a new entry stamp. Same day. Works with any current visa type. Cost: 1.5M to 7M VND plus border visa fees. Gives you another 30 or 90 days depending on what you arrange at the border.
An online visa that lets you stay 90 days. You apply on the Vietnam immigration portal, pay the fee, and get a PDF by email. You can enter on this visa at any international checkpoint. Cost: about 2.4M VND (if filed through a service). Processing takes 3 to 5 working days normally. You still have to enter Vietnam on it, which is why it pairs well with a visa run.
A plastic card that replaces your visa for up to 3 years. You need a sponsor in Vietnam (an employer, a spouse with a valid visa, or an investor visa). Not an option for standard tourists. The card means you stop needing border runs altogether during its validity period.