We need to net three states to have the majority. 2018 is the re election of Senators who won in 2012, a year we did as well as could be expected. There are no blue states with red Senators that would be easy wins - fighting just on the basis that who they caucus with is a strong winning argument.
As to the OP, it does show that there are two vulnerable Senators. I suspect that Heller is more vulnerable than the list suggests. He actually signed on as a cosponsor on the last failed terrible repeal and replace bill, that arguably was at least as bad as the earlier ones. Nevada is a purple state trending blue. If we have a solid candidate, Nevada could be a win. Flake has managed to "appear" moderate as he votes in line with the Republicans consistently. That combination might make him tough to beat in a state, with a strong libertarian streak, that is red trending to purple at best. Only if Flake is successfully primaried would we have a shot -- and that is NOT a good thing to wish for as we could get another very right wing Senator if that happens.
I have seen nothing in any article that suggests we have a shot at the Tennessee seat. The depressing thing there is that the likelihood is that we will get a worse Republican. Though like Flake, there are few votes to point to where Corker did not hew the Republican line, he has led the SFRC in a reasonable manner that may reflect his time as the second most senior Republican to Dick Lugar. (One other depressing point is that we likely get a worse chair in 2018. Jim Risch is ultra conservative and pretty unimpressive (my opinion having watched committee meetings). He might be persuaded to keep the chair of the Small Business Committee, but that would make Mark Rubio the chair -- and he has tried to make foreign policy his strength. Go down the list, there are some completely terrible choices there. https://en.wikipedia.org/wiki/United_States_Senate_Committee_on_Foreign_Relations#Members.2C_115th_Congress
A long shot to get to 51, might - impossible as it seems - might be to get NV and AZ in 2018 and to win the AL general election this year. That REALLY shows how completely unlikely - even if the country shifts increasingly more against Trump - if Alabama is a possibility!
Our problem is that the 2/3rds of the Senate that is not up in 2018 contains more potentially vulnerable seats than the group up this year. The fact that we did not get the Wisconsin seat with Feingold and the PA seat where Toomey won re-election. (The PA seat was lost in 2010 because Senator Specter was successfully primaried by Sestak after Specter switched to the Democratic party - without which we would never have had 60 votes. I liked Congressman Sestak, but Specter would have won easily. Sadly Specter then died in 2012. Rendell would have picked a Democrat, who would then have had to run in November 2014 and again in 2016. I suspect that had Sestak opted not to primary, he might actually be the Senator now. )
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