Why does aragonite form in one case and an acetate in another?
Common Ca carbonates on Earth's crust have different concentrations of Mg. There is High Magnesium Calcite and Low Magnesium Calcite.
Aragonite has in comparison to calcite high levels of Mg. 

If you mixed trusted dolomite with an acid and you took aragonite, a carbonate similar to dolomite but with lower levels of Mg, I suposse aragonite is an intermediate carbonate product in the reaction instead of the complet decomposition of the dolomite. Dolomite is decomposed with HCl at high temperatures and not as quick as calcite. So the high concentration of Mg of the dolomite should alter the thermodynamical equilibrium.

The sequence of the reaction is:


Destruction of dolomite crystals. 
Formation of aragonite. --> excess of Mg.


The Mg liberated from dolomite structure follows two ways. Aragonite accepts a low concentration of it. Then surplus Mg could be considered chemical waste while acetic acid is a chemical reactant.
In the case you get calcium acetate, acetic acid accepts Ca and it is transformed, but in this case is just a chemical reactant and his structure is not modified. Note the main difference is there is not excess of Ca.