There are quite a few generic donggles out there. They usually have some dip switches or dials on them to configure them to work with many different types of VGA type of video displays. I'm surprised that NEC didn't supply you with the apropriate adapter as that is a hallmark of many higher quality display manufactures these days.
If it were me, I'd probably go out and purchase the NEC adapter if I was worreied about quality, otherwise take a chance with a generic adapter.