Vyvanse has been significantly more effective for my anxiety than my actual anxiety medication - how does that work?

I started going on anxiety medication - Buspirone to be exact - about 2 months ago due to my stress and anxiety reaching unbearable levels. At around the same time, I was diagnosed with ADHD as well. Honestly I found the Buspirone to be pretty ineffective even after waiting the customary 4-6 weeks for it to kick in. Granted, I was on a fairly low dosage of 15mg per day.

The Vyvanse though, has been an absolute game changer. I started taking it about a month and a half ago, with 20 mg which was upped to 30mg after 2 weeks. It mitigates my daily anxiety almost completely for the most part. Sure, some of it gets through sometimes, but even when it does, I find it very manageable and something I can self-regulate as opposed to the panic attacks it used to send me into before.

So how does this work? Why is the Vyvanse so effective in mitigating something it wasn't even designed to, while the actual meds don't work that well? Is my Busprione dosage just too low?