How to make mozzarella from curd?
Most mozzarella recipes call for a combination of citric acid and rennet to produce a stretchy curd, vinegar (or a different acid) alone may not be sufficient.  Curds used to make mozzarella need to be able to melt. According to "On Food and Cooking" by Harold McGee, certain cheeses do not melt well and therefore can't be used for mozzarella:


  These include Indian paneer and Latin queso blanco, Italian ricotta, and most fresh goat cheeses; all of them are curdled exclusively or primarily by means of acid, not rennet. Rennet creates a malleable structure of large casein micelles held together by relatively few calcium atoms and hydrophobic bonds, so this structure is readily weakened by heat. Acid, on the other hand, dissolves the calcium glue that holds the casein proteins together in micelles, and it eliminates each protein’s negative electrical charge, which would otherwise cause the proteins to repel each other. The proteins are free to flock together and bond extensively into microscopic clumps. So when an acid curd is heated, the first thing to be shaken loose is not the proteins, but water: the water boils away, and this simply dries out and concentrates the protein even further.


Additionally, some pasteurization processes used with milk intended for drinking can adversely affect its ability to for a curd cohesive enough to be heated and stretched into mozzarella.