I've been keeping an eye on my vault, I collect interest every day, and I always notice that it claims an 'increased interest for 24 hours' each time I do this however the percentage never seems to change from 3%. As well, I used a Morty Card today to increase my interest rate for a month and it still remains at 3%. Am I missing something? Does the 3% never change?

It's broken, so no it doesn't drop. It was supposedly "fixed" awhile back, but it wasn't. There's a thread in Problems & Bugs but I don't think it's been paid much attention to since the "fix"

Ah, that's a real shame... I guess that was a bit of a waste of that card then, whoops! Thanks for the feedback.