I recommend using a sweet good quality wine vinegar, such as Sweet Muscatel Vinegar or Sweet Pedro Ximenez Vinegar. A good brand from Spain that makes these vinegars is Los Villares and can be purchased in various stores in the US.
The reason a Citron has to be used for this recipe, and not a different citrus fruit, such as a lemon, lime, or orange, is because Citron is the only citrus fruit that has a very thick pith and a very small flesh core. All of the other fruits have a very thin pith (unusable) and are mostly made up of eatable fleshy core. Hence, this recipe is specifically made with Citron, since it utilizes the pith.
Sliced Citron showing the thick pith