Although many opponents of Romney demanded to know why Romney wasn't being vetted like the other candidates were, the reality is that Romney was the only candidate who already had been vetted. The key test in determining whether a candidate has been vetted is if the average voter is already aware of the candidate's weaknesses. Even before the primary season began, voters were quite familiar with Romney's vulnerabilities.
A good contrast would be the difference in the way voters perceived Newt Gingrich and Mitt Romney in early December, when Newtmania was at its peak. The conservative credentials of both Gingrich and Romney are suspect because each has flip-flopped on various issues. Gingrich was even an early supporter of the individual mandate for healthcare. But at the time, the average voter only knew about Romney's flip-flops, not Gingrich's. Until the barrage of attacks was unleashed against Gingrich, voters assumed he was a pure, Tea Party conservative. When voters learned the truth, Gingrich's numbers took a nosedive. Gingrich (or other candidates) couldn't do the same thing to Romney that he had done to Gingrich, because voters already knew the truth about Romney. In other words, Romney had already been vetted.
That doesn't mean attacks against Romney are completely ineffective. He can still lose support in response to a good negative ad, and he can still be forced into an awkward situation in a challenging debate. But it does mean that Romney's numbers are more likely to be stable--just as Romney's opponents, having now been vetted, are not likely to crash a second time.
It is interesting to note that Romney's opponents, particularly Gingrich and Perry, have now begun focusing their attacks on Romney's tenure at Bain Capital. Perhaps the most fascinating part is what they're not doing: They're not focusing the attacks on Romney's flip-flops or Romneycare. Why not attack Romney's Achilles' heel(s)? Romney's opponents may have come to the realization that they're not going to take Romney down by repeatedly saying what voters already know. If so, it's a tacit admission that Romney was already vetted and survived.
As for the Bain Capital line of attack, it's something of a desperate move. Until recently, Romney's opponents gave Bain little attention. During the September debates when Rick Perry squared off against Romney, Perry attacked Romney on many things, but not on Bain. Indeed, during one debate Perry even conceded that Romney created many jobs at Bain, but simply argued that his own job-creation record as governor of Texas was more impressive and relevant. In addition, Romney was attacked in a lot of different ways during the 2008 presidential primary, but his opponents pretty much left Bain alone.
If Romney's opponents in the 2008 and 2012 primaries didn't see fit to bring up Bain, it's probably because they judged it would be a weak attack. Those who criticize Bain (especially Gingrich) have already received backlash from conservatives--including some, like Rush Limbaugh, who do not favor Romney. Many conservatives feel that the attacks against Bain resemble attacks against capitalism from the left. The direction of an attack can be important. Recall that during the September debates, Perry lost a lot of his Tea Party support when he said that it was heartless to oppose in-state tuition to illegal immigrants. If a candidate sounds like he's coming from the left, the right turns off. That's a poor strategy for those opposing Romney, since the Anti-Romney voters are the ones who think Romney isn't far right enough.