There's debate whether Vermont can go 86%+ to Bernie to make Hillary non-viable. I'm wondering if this has ever happened in a nomination contest.
The modern primary system for elections began in 1972. Of these, you are only talking about the Democratic primary process (viable is a 15% threshold) as the viability issue is not the same on the Republican side of things. Of these years, you only had contested Democratic primaries in 1972, 1976, 1980 (if you are being generous), 1984, 1988 and 1992. 1996 was not contested (Clinton reelection), and anything else is less than 20 years.
So lets look at those contests:
1972
Nominee: McGovern
Home State: South Dakota
Share of Home State Vote: 100%
South Dakota did not vote until June 6, 1972. By then, McGovern was already the nominee. While no one else was viable, it did not matter, as SD was the 48th state to vote.
1976
Nominee: Jimmy Carter
Home State: Georgia
Share of Home State Vote: 83.44%
In 1976, Jimmy Carter was the only candidate to break 15% of the vote in Georgia. The next closest was Wallace will 11.46%. Again, Carter was already the front runner, so this did not effect the race in any meaningful way.
1980
Nominee: Jimmy Carter
Home State: Georgia
Share of Home State Vote: 88.04%
This was a weird situation. Sen. Ted Kennedy challenged sitting President Jimmy Carter for the nomination. Carter ended up with about 2/3 of the delegates to the DNC, and Kennedy had no real strength outside the Northeast and the Southwest.
1984
Nominee: Walter Mondale
Home State: Minnesota
Share of Home State Vote: UNKNOWN
This was a caucus. Exact vote counts were not recorded.
1988
Nominee: Michael Dukakis
Home State: Massachusetts
Share of Home State Vote: 60%
This was a closer race than most, but while Dukakis was not the only viable candidate, Mass. was one of 30 states (plus A.S.) that he won, making the other candidates most irrelevant.
1992
Nominee: Bill Clinton
Home State: Arkansas
Share of Home State Vote: 68.05%
Bill "Come Back Kid" Clinton won Arkansas, and no one else was viable. However, this was the tail end of the primaries (AR being the 42nd state to vote) and did not matter in the long run.