I would say the effect sticks to the creatures who were near the Sorcerer at the time the vulnerability was tossed out, and doesn't travel with them. So in your specific case, even though the Sorcerer isn't around, you'll still take double piercing damage. This also means that the Sorcerer running up to an enemy who wasn't effected will not make them vulnerable either.
The reason I'm saying this is because of the wording of this spell is fairly consistent with many other spells and effects, and is in the general form: [target] [effect] [duration].
Target here is "you and anyone within 30ft of you", the effect is "gains vulnerability to Piercing" and the duration is "the next minute".
If the idea was that this effect would travel with the Sorcerer, it'd more likely be worded as something that only targets you, but has an effect on anyone near you.
I might expect a wording more like "You are affected by [fancy name]. While affected by [fancy name], you and anyone within 30ft of you are Vulnerable to Piercing damage. This effect lasts for one minute.".
Alternatively, I would have expected the word "aura" to appear within the description of the ability, which does exist with Paladin auras for example.
"For one minute, you radiate out a 30ft aura that gives all creatures within it (including you) Vulnerability to Piercing damage".
You can see the same kinds of descriptions in spells like Antimagic field ("Until the spell ends, the Sphere moves with you, centered on you.") or Spirit guardians ("They flit around you to a distance of 15 feet for the Duration.").
Lacking either of these makes me think it just defaults to what usually happens with effect descriptions; you decide targets at the moment of "casting" and then it hits only those targets for the duration.