There are some people here who trust Breyer's vanilla and some who trust Blue BELL (not "Bunny") vanilla...
but you should probably be aware that no US manufacturer runs dedicated lines-- so any ice cream that you can buy in a store will be run on shared lines with tree nuts and peanuts.
Unfortunately, this doesn't really help in terms of decision-making. Sorry about that. We, personally, do not trust manufacturers to be able to deliver a low-enough-risk product under those conditions, no matter how good their intent. Examine recalls for a few months back, and it becomes pretty obvious that ice cream is a VERY high risk item.
Soft serve from Wendy's is generally safe. Soft serve in other establishments may or may not be, depending upon how other products (mix-ins, etc) are handled and whether or not there is contamination occurring. (Does the spout touch mix-ins-- ever?)