Nice one Cath
All drugs can have side effects, and luckily you obviously watch very closely.
Did your vet say it was definitely Vidalta, or was it just coincidence?
Alternatives for hyperthyroidism include other tablets; there is some special food from Hills; Surgery; specialist radioactive iodine treatment; and even some liquid you can apply to your cat's ears daily.
Don't forget to ask your vet to monitor kidney function - it can cause problems when treating hyperthyroidism.
Good luck