Who drove Vicki to Widows Hill at the end? I don't think it was Angelique because uniting Barnabas with Josette would have been the last thing she would do. But what if Josette had led Vicki there? She had led Vicki places before. The jump from the hill may have been what was needed to motivate Barnabas to save Vicki the only way he can.
I don't think that Angelique drove her there. If she had, I think that Vicki would've jumped long before. Vicki ran off (in fear?) when she saw Barnabas burning, but then a LOT of time passed after that. Barnabas went to Angelique to give an ultimatum to make him human again, and she trapped him in the coffin. 20 minutes passed, and Laura instructed David to find the coffin. Meanwhile, the cannery was burning to a crisp. They all eventually went back to the house, along with half the town, and had a huge knock-down, drag-out with Angelique. And THEN Barnabas went off to find Vicki heading toward Widow's Hill. The timing just doesn't fit, and if Angelique was destroyed by then, then I wouldn't think that any control over Vicki would still hold true.
But I don't really think that Josette was possessing her, either. On my second viewing, I noticed that Vicki was very lucid at the end. She was looking over Widow's Hill, but I don't think that she was preparing to jump just yet. Barnabas came up to her and they embraced. But then Barnabas says something like he thought he'd lost her, and she told him that he already had, because she lives in the light and she will grow old and die, and he will not. She sounded very sane, calm, and logical. I don't remember the exact exchange but I think she was challenging him, because he said something like he'd never do that (make her like him). I think that Vicki knew full well what she was doing when she jumped, and I don't think she was under anyone's spell. I think that sometime between seeing Barnabas burning and that point, she had come to understand what Barnabas was, and that there was only one way to be with him, and the only way he would change her was if her death were the only other choice.
Now, how does Josette fit into all of this? Well, I think that Josette may have guided her, and I think that in her heart she realized that she essentially *was* Josette. But I don't think that it was possession in the sense that she was losing her own self, but just that she was just realizing that Josette was a part of her all along, and that made all the ghostly visits she had since childhood and her feeling that she belonged in Collinsport all make sense to her. So maybe I'm just an optimist, but that was kinda how I saw it on my first viewing, and that feeling was only enforced seeing it again. Now, I'm not entirely sure that the film conveyed this perfectly-- I do feel as if some scenes were missing, and that there are gaps we have to fill in. How does she realize that he's immortal/a vampire, really?? All she knows is that he starts to burn in the sunlight. What is she doing during the time when the rest of the family is battling Angelique (and by the way, this is when he's displaying his powers, but she doesn't witness any of that)? If Josette speaking and explaining things to her offscreen, just as Laura speaks to David offscreen? (On the second viewing, I realized that Laura does a lot more than I first thought- she tells David where Barnabas is in the Collins mausoleum, and she tells David to tell Barnabas where to find Vicki). So I do feel like I have to just roll with it a little bit. But I kind of like the way I see the story playing out.
Your mileage may vary, of course