I have made cream cheese icing a few times whenever I have a red velvet order or for practice. It has NEVER turned out well, it always ends up being runny. A few times, i was able to get it stable enough to fill a cake, but never to ice the outside. I've tried MANY methods (adding a lesser amount of cream cheese, not adding vanilla, keep it cold etc) & done a ton of research but nothing works. I cream butter & CC first but when I add the icing sugar, it all goes downhill. If anyone else has anything for me to try.....

how loose is the butter -- how room temp is it -- don't add any liquid at all but you gotta have vanilla in there -- you can add corn starch to thicken it up -- just right out of the box -- there's another alternative to cream cheese icing for future reference -- 

https://www.cakecentral.com/recipe/61811/white-balsamic-american-buttercream

I have noticed that whenever I use cream cheese, if it's not room temperature, I can't get it firm.  Makes sense, right?  No.  It will crust like crazy but will never get firm enough to stay on the cake - something about the powdered sugar being added to cold cream cheese that just doesn't work.

Try using it at room temperature or maybe even a little on the warm side.

I have been known to microzap it very very carefully too-- not to over heat it but just to hasten the room temp thing

I've never had a problem with cream cheese frosting being to runny to frost a cake.  I do always let the butter and cream cheese come to room temperature.  Beyond that, I just add powdered sugar until it reaches the consistency I like.

It was like that for me as well, until I started using room temp butter and cold cream cheese that I would zap until the chill was just off it.  Beat the daylights out of it.  Holds up much better than when both are really soft.   

I always cream the softened butter (not at all melty) and powdered sugar (as much as I can get incorporated) together first, add the vanilla, and then add small chunks of cold cream cheese while the mixer is running and then add additional powdered sugar if needed to taste at the end. It has a tendency to be too stiff sometimes and I'll have to add a little milk so that I can get it through a piping bag. 

A little late here, but I beat the room temp butter, powdered sugar and vanilla first.  No other liquid.  It will be stiff.  Then I beat the heck out of the cream cheese (a cool room temp) in another bowl.  I add the cream cheese to the buttercream and beat JUST until they are blended and a good consistency.  The more you beat from there, the softer and runnier it will get.