In the early empire, there was no difference between aureus dies and denarius dies: the same "precious metal" dies were used indiscriminately for both denominations.
I think that was still the case under Vespasian. Vespasian used different types for aurei and for denarii in some issues, but the style was still the same and the same obverse dies could still be used for both denominations.
Or was this perhaps the beginning of the separation of dies for the two denominations, the aureus dies being broader and finer and meant only for aurei? I am not aware of die studies of Vespasian's gold and silver, but perhaps they have been done.
In the ANNONA AVG issue of the end of the reign, to which David's denarius belongs, it would appear that dies were still shared indiscriminately between aurei and denarii. The British Museum, in any case, possesses an aureus and a denarius of this type struck from the same die pair, BMC pl. 8.20 and pl. 9.1.