Johnson would probably still publicly endorse Kennedy - he was a party man through and through, at least publicly.
Although he may be forced into picking someone pushed for by McCarthy or Humphrey for party unity, Kennedy would probably pick Terry Sanford for regional and ideological balance, whereas I can see Rocky picking someone outside the box to promote party unity. In real life, Rocky and Reagan discussed an alliance against Nixon although this plans fell through. Now it's probably unlikely Rockefeller would tap Reagan as his VP, I can see him picking someone suggested by either Nixon or Reagan as part of a compromise at a brokered convention. And most likely it would be someone who wouldn't upstage him.
I see Rockefeller picking Agnew, though Romney could definitely be a possibility as well. But for the sake of regional and ideological balance, Agnew can be seen as someone who is both a conservative firebrand yet at the same time is acceptable to the moderate wing of the party.
Wallace is probably the one who benefits most from these events - this is Wallace's dream. He can appeal to moderate conservatives who were turned off by his RL campaign because Wallace can make the case that both nominees are virtually identical on almost every issue, including Vietnam. I can see him picking LeMay, and potentially a pivot to a less extremist, Nixonesque campaign based on law and order, opposition to busing (slightly toned-down racial theatrics), and more emphasis on his "bomb 'em to the table" strategy with regards to North Vietnam.
But this election is almost certainly a republican landslide, albeit with a slightly better Wallace performance. Rockefeller was polling better than any republican candidate in a GE as of early 1968, whereas Kennedy was polling worse than Johnson or Humphrey in a general election. And a presidential debate would destroy Kennedy - Rockefeller isn't the best public speaker, but even Nixon would be able to beat Kennedy on style.
I see this as a possibility
Rockefeller / Agnew (R): 367Kennedy / Sanford (D): 85Wallace / LeMay (AIP): 86