from Sarajevo to Skopje

1

ok, first of all, id avoid vojvodina. vojvodina is nice, but in many respects its not what you might want to see in serbia if you want to experience the old serbia.
vojvodina is like visiting slovakia, czech, etc..all the former austro-hungarian provinces. its a region worth visiting, but if you would have more time and would be able to see more. monasteries etc in vojvodina are of new er date and in many ways not as important as the other ones.
i assume you wont have a car, so you could do like this to go from belgrade to despotovac and visit the monastery manasija which is wonderful and had importance as the first school, etc. its a mediavel orthodox monastery/fortress around 130km from belgrade. its also easier accessible than many others (most are in mountains and only accessible by car). you have to go to the town of despotovac (easy to reach and on the way to nis) and from there with a bus to the village near the monastery or take a taxi (its only 2-3 km from despotovac).
manasija:
http://www.serbianunity.net/culture/history/Serb_History/Monasteries/Manasija/man_pan4.html
there is a beautiful cave (resava cave) about 20 km from manasija and another monastery, ravanica (mediavel orthodox but built in a different style than manasija. mediavel serbian orthodox monasteries were built in 3 different styles depending on the year when they were built), but regretfully for both the cave and ravanica you might need a car. near the cave is also a lovely waterfall, but again a car is a must.
nis is ok, but again, i dont know how interesting. serbia is a country where places of importance are more in the countryside. cities are not really especially beautiful because many buildings were built during communism and those building types are quite recognisable.
nis is also a one day trip if you insist to stop over.

i dont know about novi pazar, but i might suspect to belgrade-nis region is better connected transportation wise.

2

actually, i do have a car:)
and Sarajevo to Belgrade to Skopje with a car seems like a long road.
I also have to say that I'm not really interested in Belgrade because I already know this city.

3

well, we havent taken this exact route (to skoplje) but from what i see at prijepolje you could turn towards central serbia rather than kosovo, which means you could take the usual route towards kokin brod, over zlatibor to uzice and ovcar banja and then from cacak towards kraljevo (where you could make a detour of a few hours and visit monastery zica and studenica, both are fantastic) and then from kraljevo to krusevac and towards nis and then to skoplje.

4

btw, studenica is situated on a mountain, in a national park. its really wonderful there and the monastery is very beautiful (and unesco protected).