I don't think this can ever be answered with complete certainty. VM could definitely stand for
venerandae memoriae but could as easily stand for VICTOR MAXIMUS. Constantine II used the title of VICTOR on his first issues as Augustus from Rome -- VIC CONSTANTINVS AVG.
However, there are examples of what seem most likely to be posthumous issues from Aquileia. There is an obverse legend from Aquileia of CONSTANTINVS MAX AVG with GLORIA EXERCITVS reverse (RIC VIII Aquileia 10, 17, 24). It seems this is Constantine I because there are also coins struck at the same time with the legend CONSTANTINVS P F AVG, which must be Constantine II. So would both legends be Constantine II or might one with MAX be a posthumous issue for Constantine I?
So, it seems possible, perhaps even highly likely, that the Heraclea issue was struck posthumously for Constantine I.
Another possibility is that this is just one more example of careless workers at Heraclea. RIC VII Heraclea 139 has an obverse legend that names Constans as junior (CONSTANS IVN NOB C) when he was most definitely not a Junior.
So, no easy answer...only more questions.