Can olive oil be substituted for vegetable oil in a box cake mix?
Yes.  As far as the mix is concerned, oil is oil.  You're not reaching high enough temperatures to worry about smoke points or anything, and you're not trying to use a weird substitution (applesauce or some such).  

That said, I've done this before, so I can tell you that it may affect the flavor of the finished cake-- vegetable oil is called for because of its neutrality, but olive oil (especially the good stuff*) often has a distinct, fruity flavor.  Depending on the (quality, quantity) oil that you use, that olive-oil taste may come through in your cake.  Whether that's a bad thing or not is up to you.  

(*I and many others would consider it a total waste to use good extra virgin olive oil in a cake.  I can't speak to any health reasons you may want to substitute, but if it were me and I just didn't have any vegetable oil on hand, I'd either go to the store now or wait to bake the cake.)