I've just scanned through some threads in the big German musicians' board about using VSTi live. Some of the forum users are experienced band keyboardists, and the general trend is to say it's still to risky and unstable to rely on software on stage. As I own a Microsoft Surface Pro 3, I have considered using light VSTi, e.g. strings, in addition, but now I'm burying the idea once again after I read about all the risks and problems.
On Saturday I just had a church wedding 80 miles away where I played a movie score tune on my MoXF; I was nervous enough about whether the MoXF, all cables and speakers would be working properly - if I imagine I additionally had a VST running with a laptop, sound interface etc. with the additional risks involved, I come to the conclusion that my MoXF flash libraries and Roland SRX sounds are good enough...