A trilayer nanoimprint process was used to fabricate high-density Co dot arrays. It is shown that choosing hybrane as the top layer resist gives better dimensional control of the replicated patterns compared to PMMA. By adjusting the etching time in the transfer process the dot sizes could be tailored in a wide range. The fabricated dot arrays with different sizes and period have then been studied by magneto-optic and magnetic-force microscopy measurements. The magnetization reversal was found to occur through vortex nucleation/annihilation. Dipolar interactions were clearly identified both in arrays with constant dot diameter and variable period and in arrays with fixed period and variable dot diameters. In the case of closely packed dots vortex chain structures along the field direction can be formed as a consequence of a nucleation cascade process.