First, thanks to Kakeladi for the heads-up on how to reply as a newbie  Sandra, you're absolutely right, that would've been helpful! This is what I've been doing: 

(1 1/2 c sugar) using equivalent in NatraTaste

1/4 c flour

3 Tbsp cornstarch 

1 1/2 c fat free milk *

1/4 tsp salt

1/2 c Smart Balance buttery spread

2/3 c vegetable shortening plus enough SB     spread to equal 3/4 c total **

1-2 tsp vanilla

* I've also used reconstituted nonfat milk powder; no alteration in final frosting results

** through math and trial, I hit upon this as the best ratio to mimic the 80% fat content of regular butter, using what we keep on hand (#creativecovidcooking  )

As to the NatraTaste, it is artificial--ingredients list says dextrose, maltodextrin, aspartame. It comes in packets only, similar to Equal, where 1 packet = the sweetness of 2 tsp sugar. I have to keep it sugar free due to family health conditions; several have severe GI intolerance to Splenda, and have found stevia to have an unpleasant aftertaste. Everyone seems to like the NatraTaste for general use, i.e. in coffee; I also have great success with it in my recipes, especially my sugar free cakes and pies--no weird taste during or after with those things. Which is why this frosting has me so stumped! 

 I am not familiar with either NatraTaste or Smart Balance spread.  It does sound to me as though the slight sour taste may be coming from one or the other of these or even a combination of the two.  I have baked with Stevia and as long as I combine it with a small amount of real sugar, I notice no off taste, but when used alone (in baking), I do think it has a weird undertaste. 

It might help to add more vanilla flavoring.