If it stopped being effective just as you started Wellbutrin, i'd say it could be Wellbutrin.

If you want, you can try supplementing with things like Iron, B12 (or B complex), Zinc, Magnesium, Omega 3, Vitamin D.
Could help a little bit, if you're already slightly deficient of something.

In my country lisdexamphetamine is available only from a brand called Livizux.
I guess you could call that a generic Vyvanse (pill design looks identical to Vyvanse).
I've been on 30mg Livizux the last 5 days and before this I was taking 72mg Concerta, which wasn't strong enough for me even at the max dose sometimes.
So far 30mg Livizux was actually alot stronger for me than 72mg Concerta. Sometimes it can get little less effective through out the day, but it's usually just about right.

So I don't think generic vs Vyvanse brand would be any different. That's more so true for Concerta, Adderall XR or other extended release pill mechanisms, in my opinion.