First, try to get your punctuation right: 'relationships' is a plural noun - no apostrophe. Second, is the question yours or one set by some teacher/tutor/instructor/examiner? On the answer depends how critically I'd advise tackling it, since in the way you word it, it seems to presuppose all sorts of things which aren't so. And I can't believe any reputable pedagogue would use " 'the' 19th century,upper class etc" rather than "19th-century upperclass etc" (note the correct inclusion of the hyphen and the elimination of the extraneous comma). That being so, try and reproduce any essay question accurately in future.
Now let's proceed to the meat of the matter. I assume you mean Thackeray's novel VF. (Bunyan's VF doesn't deal with the 19th century.) In future, please say so. As elsewhere on this website, unless the question is thought out properly before being put into words, and as much useful information given with it, the person providing the answer may well be puzzled, hence give an unhelpful response.
VF isn't 'about' the difficulties of personal relationships, particularly marriage ones etc in the sense that they're a central theme. It's 'about' much more. It wouldn't be so very great a novel if it weren't. It certainly features many tricky marriages. That said, not all the marriages in it are 'tricky'. The O'Dowds', for instance, is happy, even though Mrs O'D is presented as absurd and her husband Mick as hen-pecked. (Perhaps because Thackeray could only conceive of a marriage as 'happy' if the partners were comic; after all, his own wife was a lunatic.)
If VF is 'about' anything it is that overwhelming preoccupation of Thackeray's, snobbery - a subject on which he is the best novelist in English literature. Now snobbery and its ancillary activities, notably social-climbing, often, though not invariably, lead to unhappy marriages. And some of the factors involved along the way, such as the deception and ultimate ruin of one's creditors, as effected by Becky and Rawdon Crawley, were more typical of the 19th than of the 21st century. But they weren't exclusive to it. One might argue that the Rawdon Cs' marriage broke up because of their lack of money, a lack caused by Rawdon's rich aunt's leaving her money away from him to his elder brother Pitt. And since the aunt's aversion to Rawdon's marriage arose in great part from his 'throwing himself away' on the relatively low-born Becky, snobbery played an important part But even that is not the whole story since Becky's brains and Rawdon's stupidity were incompatible long-term whatever their prosperity.
And VF isn't exclusively about upper-class English society (again note the preferable hyphen). The Osbornes and Sedleys are from the mercantile upper-middle classes to start with. The Sedleys through bad luck in business sink to shabby genteel middle-class status. Even among the true upper classes there are subtle variations of status, with Lord Steyne being as greatly 'superior' socially to a provincial baronet like Sir Pitt Crawley as Sir Pitt is to the Osbornes.
Thackeray does duck showing us any wholly satisfactory relationships. Would Dobbin and Amelia's marriage have been a success? We don't know. Thackeray buries it under a 'happy ever after' coda.