Is it 2/3 of the entire Senate, or 2/3 of those present?
It's 2/3 of those present, as long as there is a quorum (51, the minimum number required to be present to conduct business). The Vice President only votes in the event of a tie vote. So, with the soon-to-happen 50/50 split in the Senate (50 Republicans and 50 Democrats), a quorum can be established by all 50 democrats and just one Republican showing up.
Assuming all 50 Democrats vote to convict:
If all 50 Democrats and 1 Republican show up, a 2/3 vote is 35. The Republican is free to vote no and conviction would still happen.
If all 50 Democrats and 5 Republicans show up, a 2/3 vote is 37. The Republicans are free to vote no and conviction would still happen.
If all 50 Democrats and 10 Republicans show up, a 2/3 vote is 41. The Republicans are free to vote no and conviction would still happen.
If all 50 Democrats and 25 Republicans show up, a two-thirds vote is 51. All but one Republican are free to vote no and conviction would still happen.
If all 50 Democrats and 40 Republicans show up, a two-thirds vote is 61. All but 11 Republicans are free to vote no and conviction would still happen.
If all senators show up, a two-thirds vote is 67. All but 17 Republicans are free to vote no and a conviction would still happen.
This sets up some interesting possibilities. While a lot of Republicans may be secretly willing to vote to convict, all of those dread the backlash that would follow. But if just one Republican is willing to do nothing more than show up to establish the quorum, he or she may be the secret hero to all others. The one can abstain or vote no. The others can "boycott" the vote on principle, thereby allowing Trump to be convicted, while leaving themselves political cover to get through the next election. Also keep in mind only 20 of the 50 Republican senators are up for election in 2022. That puts the election of the other 30 far, far into the future, politically speaking.
Trump's influence is unlikely to last far, far into the future. By that time, there will be a dozen Republicans wanting him out of the way so they can mount their own presidential run. Convicting Trump in the upcoming trial may prove to be far easier than many expect.
It may be happening behind the scenes right now that the Republican senators are talking to each other, looking at their maps and calendars, and developing their way to convict Trump without paying a high political price.
I had not thought of that strategy until Turtle mentioned it. Thank you, Turtle. Fun thought!