I don't know about vault/IP tradeoff, so someone else should answer that.
On splitting: Eventually for most students, it comes down to the people and firm culture between similarly-prestigious firms. In order to find out which firm "fits" you, it's best to work for the firms you are most interested in. So firms know that it's advantageous to students to split, and it's generally accepted. I don't think it sends a particular message to a firm except that you are interested in another firm. But that applies to almost all of their 1L and 2L summer associates. As a 1L though, I'd be sure to really put in the effort and maximize my time at each firm -- especially if the other 1Ls aren't splitting -- if you're interested in returning.