By implementing higher taxes on the extremely wealthy and higher taxes on big corporations
Sanders introduced a similar bill in 2014 and received no cosponsors or support in congress. Virtually the same bill in 2017 now has 20 senate Democrats on board as cosponsors. So to answer your question, no, I don't think Clinton would have vetoed a medicare for all bill because she never would have allowed it to reach her desk. Just like Jerry Brown did in CA. There would be Dems in congress, just like there were under Obama, people you support, like Joe Manchin, who vote against it and it would never have reached the 60 vote threshold under a Clinton administration. The massive influx in popularity towards a medicare for all system like the one Sanders is proposing is directly attributable to how badly Trump and the GOP congress have totally bungled the ACA repeal. Americans across the political spectrum can see the contrast for themselves, in real time as the GOP try to kick millions off of healthcare and give huge tax breaks to their wealthy donors while Sanders and the Democrats are trying to ensure everyone is covered and it's affordable. The timing of releasing the plan couldn't have been better.