Given that being the sitting VP is considered a strike against you,
That's silly. Vice Presidential nominees are almost always frontrunners for their parties nomination in the future, if they want it. Over the past generation Dole, Mondale ,George H.W. Bush, and Gore received their party's nomination. It's certainly a better springboard towards the Presidency than continuing as Representative Ryan. Who was the last representative to receive a major party's nomination?
I would note that only one of those men actually won the presidency, and that a couple of them are considered rather dismal failures, punchlines even, as candidates (Mondale and Dole, to be precise, though Gore might also slip on to the list depending who you ask.) Dole had also had nearly two decades for voters to forget he had once even been nominated as VP, but I'm unsure what that really means in this context.
The best springboard to the presidency has historically been being Governor of a successful state, which may or may not have been Ryan's next step were he really interested in being president. Being a well know Representative would certainly help with that, eh?
Ex ignorantia ad sapientiam; e luce ad tenebras
I don't think its that clear cut that being a governor of a succesful state is the best springboard. Over the last 50 years or so, I tihnk we can conservatively estimate their have been at least 100 governors who most would agree were successful. Out of that number, only Reagan, Carter, Clinton, George W. Bush, Dukakis and Romney have received their party's nomination. In recent history , only a very small percentage of sucessful govrernors have made the jump to receiving a presidential nomination.
In contrast, Kennedy, Nixon, LBJ, Hubert Hunphrey, Ford, Dole, Mondale, G.H.W. Bush and Gore received a party's nomination after being nominated as V.P. That's nine eventual nominees out of less than 25 that have either been nominated for V.P. and lost like Kennedy, or run for the Presidency after serving as V.P. like the first Bush. For someone like Ryan, the odds of receiving the Republican nomination in the next couple cycles are infinitely higher if he runs runsg as V.P. as opppsed to waiting for Scott Walker to exit the governor's office, winning and election as governor and then creating a favorable record as governor. If Romney loses, Ryan will be the odds-on favorite to win the Republican nomination in 2016, assuming he handles himself credibly in the campaign. Palin received about as much negative press as possible last cycle and still would have been a formidable challenger for the nomination this year if she had tried for it. If she had stayed as Governor of Alaska in 2008, would she have even qualified for the debates if she tried to run this time around?
Being thrust into the national spotlight has risks as you point out. Its easier to make fun of a V.P. because even hard core partisans aren't that invested in them. No comedian will make of Barack Obama, but democrats will laugh at crazy Joe Biden jokes. But V.P. nominations mean national recognition and the chance to travel the country campaigning in a spotlight. Governors have a hard time generating name recognition outside of their own state, V.P. candidates don't have that problem when they eventually run on their own.
Kennedy was nominated for VP spot with Adlai Stevenson in 1956, but lost out to Estes Kevaufer. So, unlike the other people on your list, he never actually ran for the office and thus was not implicated in any losses that might have happened. LBJ got into the White House by succession, as did Ford, so even sitting in the Oval Office is only a 50/50 shot.
Ah, I see, we're looking at the numbers in different ways. I'm looking at the people who won, not just ran or even got nominated, and that yields a whole different outlook. If we pick 1960 as the beginning of the "modern" campaign, not too controversial I hope, of the eight men who ran and won, five of them were governors, covering 34 of the last 52 years (1). Running for president is easy, and even fun if you have enough money; actually winning is a tad harder. I believe an ambitious man would look at the history and see a clear template for victory.
I think you underestimate just how much baggage a sitting VP has to carry. Voters are likely to attribute all your successes to the president, but you'll still have to answer for all the failures-- think about Gore and the Elian Gonzalez fiasco. In the two situations where it has worked (LBJ and Bush/41), both winners largely ran as mere continuations of popular presidents, which cleared out the typical friction-- in short, they were the exceptions that prove the rule.
"Ryan will be the odds-on favorite to win the Republican nomination in 2016." I don't share that optimism. Given the absolute frenzy Republicans have shown toward Obama, I find it hard to believe that anyone connected to losing in 2012 will have any kind of national career for about a decade. And Ryan doesn't strike me as the kind of guy who's going to "go rogue" and use a sinking ship as a soapbox. YMMV, as your comment about Palin clearly shows :)
(1) If you take Kennedy and Johnson out as being still too much of the machine politics era, the numbers skew even stronger for the govs. When to think modern politics really begins is a hard question to pin down; I selected 1960 because of the Nixon-Kennedy debate, but you could look at Chicago 1968 and make a case that it took a long time for the old systems to fade appreciably.
Ex ignorantia ad sapientiam; e luce ad tenebras