I don't know how much we can take from the description of the consumer survey's trailer because, as KMR pointed out:
It's important to bear in mind that it is very common for trailers and TV series promos to have brief clips of dialog strung together totally out of context, giving the impression of plot points that are actually very different from what is in the real story.
Julia's question, Barnabas' response, and Roger's reaction could be taken from three completely different scenes for all we know. Though even if they are from the same scene and they're actually seen in sequence in the film in the same way as they appear in the consumer survey's tailer, that still doesn't necessarily mean that the family believes Barnabas is literally a vampire. They could take his remark to mean he's metaphorically a vampire, just as many businessmen might describe themselves. And considering how Roger's character is supposed to be in the film, that would excite him, which would explain his reaction if it does indeed come from the same scene.
And as for Barnabas' nails, well, he could explain them as an affectation he picked up while living and/or doing business in China. The long nails of characters are often explained away that way, and not just with characters of Chinese nationality.
Actually, there are probably many ways Barnabas' pallor and nails could be explained in the film. Though whether the film uses any of them is anyone's guess at this point. The only way we might finally know is when we finally see the film for ourselves...
75 days 7 hours 25 minutes 46 seconds until the day the Depp/Burton Dark Shadows is released(ET)!!