So sorry to hear Tazzy isnt feeling too good :/ Not sure if I can help of not, but our cat Schmoo is on Vidalta and has been for well over a year.
Thyroid meds are designed to reduce the T4 hormone which is overproduced when a cat has hyperthyroidism. This hormone basically sets the cats metabolism to overdrive, which if left can cause rapid weight loss (even with them still eating) and then increased heartrate & blood pressure etc.
Its great news if they can operate to remove the thyroid, but they will need to get the T4 level down before they operate so its safer, which is why they will have given you these meds.
As far as I am aware there are 2 main tablets used currently - Vidalta and Felimazole. They work in slightly different ways. From what I understand (in laymans terms - I'm no vet LOL!) Vidalta is a once a day 'slow release' med which the cat metabolises throughout the day into the drug that will sort the T4. This may be why even after you stopped giving it, it will take a while to get out of her system?
But Felimazole is already the 'T4 fighting' drug, which is why you have to give it twice a day, as it works almost immediately so you have to spread it out?
Cats seem to react differently to both - with Schmoo we couldnt stabilise her on Felimazole (we cant operate as shes too old) so we are on Vidalta 10Mg daily.
Do you know what dose of Vidalta you are on? They should start low and then build it up, but agreed that Felimazoles doses do start lower - I think they have a 2.5Mg tab, so that would be a start dose of 5Mg a day? If shes reacted that badly though I would suggest trying Felimazole? But hopefully your vet will go over this with you when you speak to them.
In terms of eating, I think its a case of trying a variety of foods, but if you can stick to lighter stuff as the vet already said - boiled chicken works with Schmoo when shes got an upset tum? Also if she doesnt already I'd try wet food - then shes less likely to get dehydrated as well if shes not drinking?
Schmoo is currently on Royal Canin Digest Sensitive - but the vet should be able to give you some senstive tum food to try?
I hope you get on ok with the vets and Tazzy starts feeling better soon!