For a class of small data U(1) symmetric solutions to the Einstein-massless Vlasov system, we prove that they can be achieved as weak limits of vacuum spacetimes. In particular, this extends our previous work in which the weak limits are restricted to solutions to the Einstein-null dust system. This result builds towards a full classification of high-frequency limits of vacuum spacetimes, as envisioned by conjectures of Burnett. The proof proceeds by first constructing vacuum solutions which approach solutions to the Einstein-null dust system, and then taking the number of families of null dust to infinity. This is a joint work with Cécile Huneau.