For me the relationship between Barnabas and Elizabeth is one of the best things about the film. And given that apparently from the get-go the film's Barnabas never hid the fact that he was a vampire, for him it seems to make sense that he wouldn't hide that fact from Elizabeth after he's released into 1972. I mean, he didn't even hide from the children that he's the same Barnabas as in the portrait - though they didn't really believe him and were more willing to believe he was spaced out on something. At least that was Carolyn's reaction. David, on the other hand, seemed to take an instant liking to him.
And as for how the original series' Elizabeth might have reacted had Barnabas told her soon after meeting her that he was a vampire, well, we'll never really know. Though when it sunk in, initially at least, I strongly suspect she would have reacted the same way that the film's Elizabeth does: complete shock. One clue beyond that, though, could be the fact that one way in which both women are alike is that they're both fiercely loyal to family and extremely mindful of family honor. But something to keep in mind that makes them very different is that the original series' Elizabeth and the film's Elizabeth find themselves in extremely different financial situations. But I'd rather not get into that until after it's shortly dealt with in the slideshow.