"VANITY FAIR" is about the difficulties of personal relationship's,particularly marriage relationship's in the 19th century,upper class English...

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.