Fragrant, fresh herbs are a cornerstone of the Vietnamese kitchen. They're not for garnishing but rather for enjoying practically like a vegetable to add punch flavor to foods. You can have a super fresh supply of herbs if you grow it but not all of us can do that. I grow some and buy others. Just harvested herbs from your garden are easy to keep fresh: put them in a vessel of water and keep on the counter.
The herbs you buy unfortunately cannot be treated that way. Kept in a produce bag, the herbs don’t last long. They soon lose their perkiness, which for dishes like pho, diminishes the experience. I’ve been experimenting with a different way to keep herbs fresh.
If you’ve read VWK and my books, you know that I’ve favored keeping certain herbs like cilantro, mint, tia to purple perilla, rau ram Vietnamese coriander, and kinh gioi Vietnamese balm in the fridge like flowers: Cut off a knuckle’s length of the bottom stem then put the herbs in a container with 1 to 2 inches of water then cover loosely with a plastic produce bag. (The VWK Vietnamese herb primer is here, in case you need a guide.)