Well, we know that so long as Adam lived, Barnabas was free of Angelique's curse. Who knows if Adam's life actually completely guarded Barn from other forms of attack.
The thing is nothing in the DS universe seems to be as it appears at first view. For example, Quentin's portrait appeared to make him [spoiler]impervious to physical maladies and attacks and to make him immortal - yet whenever Quentin was buried alive (and lucky, lucky him - that happened to him twice), the show made it a point that there was a very real danger that he would die if he wasn't rescued in time. And until Jeb's curse, with the exception of Ang trying to reinstate her curse, which may have been a special case, nothing was ever allowed to play out long enough with Barn for us to find out for sure one way or the other whether Barn really was guarded from it because of Adam - though we certainly saw how Adam was affected by Ang's attacks on Barn. And with that in mind, I've always thought that it might have been a very real possibility that when Barn became a vampire under Jeb's curse, Adam did as well. I even began writing but was never able to find the time to finish a fanfic with that very premise as one of its plotlines.[/spoiler]