Worms reappearing despite being treated!

We now have a second kitty, 7 month old Mia, along with our lil girl, year and a half year old Humle, and they're both indoor cats who've been dewormed... But! When we travel, our girls stay with my wonderful in laws who have a garden and Humle had become an outdoor cat when she's with them. Mia's only ever been inside. Someone threw up what looked like roundworms last night- I'm 99% sure it was Mia- and I'm so confused!
Someone pooped roundworms a few months ago while they were at my inlaws', but again, my m-in-law wasn't sure who, and they were both treated. And now, a few months later, this vomit! Noooo other symptoms, other than Mia being food-crazy, but she does still seem normal- tons of energy, happy, no diarrhea.
Could these worms (99% sure that's what they were after research) just keep coming back? What am I doing with deworming meds if they're not working?

What's going on!!!

Worm eggs can remain viable in an environment for a long time. 

If your infected kitty was going outside and leaving deposits, it could be a source of reinfection every time she goes to that environment.  You may indeed need to keep up a de-worming schedule for a while.

Thank you! Any idea why she'd/they'd be resistant to the deworming meds though? Maybe I don't completely understand. Also, would it matter that Mia, whom we're 99.9999% sure threw up what definitely looked like roundworms, has never gone outside?

Do your cats groom each other?  The eggs could have been transferred in that process.  For example: Humle has worms and leaves a deposit out in the garden.  She then cleans herself, including surrounding hind leg fur, which could possibly leave eggs in the fur.  Later, when kitties are together, Mia grooms Humle, possibly receiving egg transfer at that time.  It's a self perpetuating cycle. 

Medications will kill the worms as they hatch (inside kitty's system), but there will still be eggs in the environment for a while.  Which is why you need to continue medications on a regular basis to keep killing off the eggs as they hatch. 

I'd suggest meds for both cats, and keeping them indoors.  It may take several rounds of medication to clear everything up.

Also, clean up the deposits in the garden.  Bury them well, as they can be a source of re-infection.

Be sure to use medication prescribed by a vet, as some over the counter products are not as effective.

Seriously, can't thank you enough for the information and advice. I'll call my vet first thing.

Since I live in Florida with the bugs my cats get dewormed year round to prevent heartworm.

Smart. We're in Copenhagen and, so far, only Humle's been a sometimes-outdoor cat who must be the one to keep passing it back and forth with Mia. The thing is, they were both treated just a few months ago. Maybe they need a different med?

I had a couple of foster kittens who needed about 3 courses of dewormer before they finally got free of round worms.  It can take some work to get rid of those things sometimes, with their various life stages.  I wouldn't worry too much about details- just be prepared to repeat until they're gone.