You're overthinking it.
Zombies are goopy flesh, so there's no thematic reason for them to have a vulnerability to fire damage. They already have a weakness against radiant damage baked into their Undead Fortitude trait.
If your issue is what to do with your players moving forward you can just tell them "guys I misread/misremembered, and zombies aren't vulnerable to fire". They should be understanding about it. That doesn't detract from the efficacy of their plan before. Fire still hurts, and fire traps will still cause most things to have a bad day.
Vulnerability = weaker zombies
If you give them vulnerability to fire damage, it's going to make zombies less of a challenge. Period. I don't think it's a whole step from CR 1/4 down to CR 1/8 (see below),1 but fire damage is pretty easy to come by and your mileage may vary.
1 I don't claim to be great at CR calculation, but a vulnerability does not denote an entire defensive CR step-down. Even if it did, it would only incur a total CR change of a half-step.
