Limit on amount of vanilla take home to USA?

It does not.

We brought back 4 500ml bottles.
I've been told aa long as it doesn't appear you are stocking for resale, should be not an issue.
Be sure to declare at your port of entry.
In L.A. the CBP agent said of our vanilla, coffee (8 kilos), and 2 liters of liquor-
"All the usual suspects", and waved us through. We declared $300 US

No restrictions as long as you are within your allotment. I usually bring back 250ml and 500ml and the issue is never the amount of them as much as the weight as liquids are heavy. Canada and the US are similar on this and I always bring back at least a dozen bottle. I had 20 last time.