I was thinking about this too. Since time seems to flow slower in the RoD it is possible that Ven`s heart could have stayed in the RoD for that probably rather brief amount of time. Since Ven also seems not to respond to Aqua`s call and is very passive looking, his heart was is still slumbering and healing the damage that was done with the fight against Vanitas.Let's look at which time period we are there and what might happened there that involved Ventus.
There are actually two principal events that can be worth examined in this context depending on what happened in the RoL at the same time:
The first and more likely event is of course Sora turning into a Heartless. As we know Sora's body vanished and reappeared as Roxas in Twilight Town. Ventus' heart is heavily implied to remain with Sora's body and thus, during the time between Sora's sacrifice and Roxas' being fully created it is certainly possible for Ventus to make an appearance in the RoD.