Vukovar is in the Eastern part of Croatia; it is vastly different from Western Croatia bordering the Adriatic Sea and home of
Dubrovnik, which is one of the most beautiful historic cities in the world.  (We stayed nearby at Svetti Stephan in the 80s.)   
Vukovar's population was about 1/2 Serbian, which led to the  virtually total destruction of the city when the Croats and Serbs
fought over how to breakup Yugoslavia in 1991. We skipped the city tour and instead went on a wine tasting about an hour
away in the town of Ilok.  Thereafter, we boarded the boat and sailed to our first stop in Serbia: Novi Sad.
Vukovar, Croatia And Novi Sad, Serbia
Next is Belgrade, the capital of Serbia, or Home
The outskirts of Vukovar are
quickly taken over by vast
vineyards; this is one of Croatia's
largest grape growing regions.
The Principov wine estate is
surprisingly modern-looking hilltop
destination resort.  The
surrounding views are almost
unlimited.
View of the town of Ilok a
couple of miles distant
Our best quality tasting experience was in the wine
caves of Ilok, where the Principovac wines are
stored .  The grapes are local varieties, which
produce both red and white wines.  The caves are
filled with bottles that have been stored for 50
years or more, including whites (they have to be
recorked every 15 years).  A White desert wine,
which is one of their specialties, called a
"Coronation" wine was served at Queen Elizabeth's
coronation in 1947.  I bought the current 2016
vintage for later consumption on our boat (click on
thumbnail).  It was outstanding!
Past Ilok the Danube flows for 125 miles solely within Serbia.
Serbia emerged from the breakup of Yugoslavia in 1991 ,and
subsequently the war with Kosovo in 1998, to become an
independent state in 2006.  Novi Sad is Serbia's second largest
city with a population of 250,000.  The city center retains very few
buildings predating the 19th century because it was decimated in
the 1848 Hungarian invasion and further damaged by NATO
bombing  during the 1999 Kosovo war.  Novi Sad looks like a fairly
modern city now, at least around the city square.  Evidence of
modernization is seen in the sign at left, which includes both
english and cyrillic names.
One of 3 bridges rebuilt after
the NATO bombing
The modern concert hall.  Click on
thumbnail to see program
announcement
The city square
Below is an apartment block built during the Communist
era.  It is typical of the drab and now run-down architecture
seen throughout the Eastern European countries
.  I didn't
take this picture; it was taken in Sofia and found on Google.
Across the river: The fortress of Petrovaradin,
which dates back to the 13th century