I have a problem.. I have to make 21 batches of buttercream using wiltons recipe for buttercream. I thought I would save some money and bought a 50lb box of vegetable shortening (used commercially). But when I mixed it with the vanilla and the water...it would not mix in (cream). That mixer was on for 20 minutes.

I went ahead and added the powdered sugar and meringue powder anyway and it seems to have come out ok, but I am afraid that the icing will break down later.

I thought about adding a tablespoon of powdered sugar at the creaming stage to see if this will help promote the shortening to cream...but I don't know... Does anyone have any suggestions?

you should be fine, but you really should add the liquids in AFTER the powdered sugar.

Powdered sugar is always the last thing I add. I put in all the shortening, water, flavorings, etc. and 1/2 of the powdered sugar. I mix it on low, and when there is no more loose powdered sugar, I turn it up to medium high to cream the shortening. I've never creamed just the shortening and water as fats and water don't really mix well and something needs to absorb the liquid. Your buttercream won't break down from this. The point of creaming is just to get the lumps out of the shortening.