There are many factors behind this victory by Vinis.  For one, she started her campaign 4 months sooner.   Clark waited too long to announce and start his campaign.  He had to play catch up.

Secondly, Vinis had the full support of the Democratic Party of Lane County.  They are a powerful organization that knows how to get out the vote for Democratic candidates.  

I also believe that a key factor is that many Republicans didn't bother to vote, since the Republican Presidential primary was already over.  In contrast, the Democrats had a very tight race for President, which no doubt resulted in a high turnout for Democrats.

Finally, I think that Clark ran an awful campaign. Vinis  strongly attacked Clark's record on a number of issues.  Yet, Clark did not offer either a strong defense to these attacks on his character, or go on the offensive and attack her at all.  Clark was way too much a nice guy in the campaign.

And the simple fact is that attacking your opponent wins elections.