"She accepted with glee," a source shared with the mag.
If you're worried that Ian and Nikki are getting engaged too soon, don't be -- their choices literally don't affect you in the slightest. Except for the whole "Now I'll never marry Ian Somerhalder!!!!" thing. That's pretty major.
Besides, another source told Us that: "The groundwork had been laid before. They had always been attracted to each other, but always in another relationship. It may seem quick, but they knew it was right."