I was going to say what MM said.
They are completely different, but can be linked together. If you are able to take the deja vu which is usually from a brief dream/vision (at least in my case), and remember it, then it'll become a premonition. But usually deja vu is when the remembrance of such a thing is brought up when the event is happening. Thus it can't be a premonition, you didn't remember it until it was too late.
Deja vu can come in two forms. Having two very similar situations happen, like having the same conversation with someone twice. One might think that's deja vu. To me... deja vu is dreaming of something that'll happen, then recalling the dream the moment the event is happening, after having completely forgotten it.
I get deja vu frequently. It's never anything big, but when something little is happening, I'll recall having a dream about it. The longest its lasted with me was about 3 seconds.