The 'Vampire Diaries' actor is set to marry the 'Twilight Saga' star, who recently finalised her divorce from Paul McDonald, after just six months of dating.
A source said: ''It happened over the holidays.''
The couple were overheard discussing their plans to have a small summer wedding at the World Dog Awards last weekend.
The source told InTouch magazine: ''They mentioned a June [wedding] date. They're really excited.''
The 26-year-old actress has been wearing two diamond bands on her right ring finger for the past few weeks, and the insider explained: ''Nikki said she wears the ring on her right hand because they want to keep it quiet now.''
Representatives for the couple declined to comment on their relationship status.
Nikki announced her split from former 'American Idol' star Paul last March and stated they had been living apart for six months.
The former couple got married in October 2011 after just seven months of dating.
She started dating Ian last July and they celebrated Christmas with both of their families in Idaho last month.
The 36-year-old actor, who previously romanced his co-star Nina Dobrev, recently gushed about the brunette beauty, calling her one ''of the most beautiful ladies in the universe.''
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