There will always 100 different versions to all things "Dallas", especially re Victoria...When she left, she was already working on a few scripts to produce and/or star in, all of them for TV. I´m sure she wasn´t content with her character because she told that in a lot of interviews, including those published in Ultimate Dallas and Dallas Fanzine. She said that all she had left to do as Pam was "becoming Mother Teresa", in her own words. She loved and cherished the character but she had enough of it by her last season.
Anyway, we´re talking about the infamous soap producers of the 80´s, and there´s always been an alternate story about why a star left a show. There´s the version that, in order to be let go, she asked the bosses to get paid as much as Larry Hagman, but didn´t get it (maybe because she knew they wouldn't?). I´m also sure that VP was already with her "Principal Secret" in mind so she had her hands full so to say. And also writing work-out books...and being married to her millionaire plastic surgeon (no joke here please...). Whatever the "truth" is (and I myself don´t need to know that "mystical" truth), maybe it´s a combo of all things mentioned above...Anyway, she never returned to "Dallas" and even stopped watching it after her departure.
About being believable and realistic, I think it depends on all elements involved, but very especially on the producers. In the supersoaps´ case, it´s obvious that the Lorimar shows tended to be a bit more "realistic" because they filmed often in real exteriors, and they hired actors trained and experienced in theater and movies (even "Falcon Crest", which probably was the least "realistic" of them). The Spelling productions had the vicious habit of using good actors and wooden younger people (not all, but most of them).
I think the problem could be handled when the producers got along with everybody. This happened on "Dallas" right after Leonard Katzman and the Duffster left. Then the Dream Season began to "dynastify" everything and Larry got pissed off. As we know, it never really recovered from that despite Katzman and Duffy´s return. "Knots Landing" always was a different beast because the ensemble cast was consistently solid and capable to save some repetitive scripts. But when Jacobs left and the new producers came, along with the production cuts that had Harris and McCashin fired, the show resented that too in the next years. Acting kept being great but plots were less believable than usual, in general.
What seemed to happen with "Falcon Crest" is different. For a start, it never aspired to be as "believable" as the other Lorimar soaps and featured more violence, grittiness and gothic atmospheres than the other two. But I think we all here also agree that whatever the first three years had, the rest of the show lacked. And again, this had to do with the producing team, fired after Season 3. There were revolving doors in the next years, and soon the show lost any chance to be qualified as "realistic" and much less, "believable". But it was as fun as hell, and in a sense, no other soap had such an ability to deliver constantly suspenseful cliffhangers, even every week, and all this up to the boring Season 8, when the show was sentenced to death and seemed to be plugged to a breathing machine, like Angela the next year...
I´ll let others speak more extensively about realism and believability on the Spelling soaps, which I have watched but not adored, as many of you must know. However, Miss Piggy is a big fan, and even re-enacts the Alexis dialogues when she is in the bathroom. I think that even once she re-enacted the Ritas catfight!!