Assuming that Kasich would accept a VP slot with Trump, it would no doubt increase their chances of winning Ohio, but don't be mistaken, Ohioans will recognize the name at the top of the ticket and the one under or after it will not hold the same weight if it appeared at the top. Second, Trump is ahead in Florida right now for the Republican nomination but that isn't the same for the general election. I would argue that Florida will be an incredibly tough sell for Trump given his stance against illegal immigrants and the fact that Obama won the state in both '08 & '12. Lastly, go back and look at the last two presidential election results. Obama won both Ohio and Florida but could have lost them both and still won the election. Since the democrats have added Nevada, New Mexico, Colorado and Virginia as solidly blue, the Republicans have to actually start winning states like Wisconsin, Michigan and Pennsylvania......which are historically blue also. Go to 270towin.com. There is an interactive electoral map there that you can change around any way you want. In challenge you to find a path to victory for Trump given the historical trends from each state.
http://www.270towin.com/maps/Mm2rj I went ahead and did the math for you. I took away Ohio, Florida and, for good measure, Virginia from the Democrats and they still win with 272 delegates. Having said that Obama beat Romney by 3 pts. in Virginia, 2 in Ohio and 1 in Florida.
They could always just rig the election like the dems always say they do. Maybe Hillary will have Trump Vince Fostered, we can only hope