This is the most important step. Quickly cover the drain opening with a plug or a tight-sealing cloth. By sealing it, you trap the expanding carbon dioxide gas inside the pipe, forcing the pressure downward against the clog instead of letting it fizz out into the sink.
Here are several angles and "good features" that make this topic compelling: unclogging drain with vinegar
For stubborn, slow drains, repeat the entire process 2-3 times. If no improvement is seen after three attempts, the clog is likely solid (grease, hair mat, or an object) and requires a mechanical solution (plunger or snake). This is the most important step