Governors: Dewey, Stevenson, Carter, Regan, Dukakis, Clinton, Bush 43. 4 out of 7 won, 57%
Senators: Kennedy, Goldwater, Dole, Kerry, McCain, Obama. 2 of 6 won, 33%
Sitting VPs: Nixon, Humphrey, Gore, Bush 41. 1 of 4, 25%
As you can see, being the sitting VP has little advantage comparatively. If you throw in Mondale and Nixon's second run then the stats improve slightly but your chances are still better if you're a Senator. Of course if you include Truman, Johnson, and Ford the stats are somewhat better than being a Senator but are still worse than being governor. But those three were all sitting presidents when they ran so as I said before, they don't really count. Bottom line, being the VP holds no real advantage over other high-profile elected offices.
But you're only counting people who actually got the nomination. Lots of governors and senators ran for president and never even got the nomination. Others probably *wanted* to run, but were deterred because they didn't think they had a large enough national profile to give them a shot at the nomination (whereas they might have had a shot if they'd been VP).
Also, sidenote: You're comparing apples and oranges by limiting the VPs to *sitting* VPs, whereas in the governors category you count people who were *former* governors at the time they were elected president (like Reagan). Why include the "sitting" requirement? Four of the eleven VPs we had between Nixon and Cheney ended up as president at some point (whether by election or by taking office when the president dies or resigns). Two others (Humphrey and Gore) came so close to being elected president that a minor change in circumstances would have gotten them there.
On the flip side, we've had hundreds (thousands?) of governors and senators over the last 50 years, and only a handful of them have made it to the White House. All things considered, being VPOTUS seems like it offers the best odds at getting you into the Oval Office.