This is a major plus in the native iPhone music player. The songs transition into each other seamlessly without breaks in the sound. There is a small hesitation between tracks in the Vox player. Personally I think this is a very important feature to have since a slight disruption in the sound can be an annoyance.
Seamless Track Transitions
Isa, gapless playback is supported by VOX for iOS by default, however, if played from network directly, it depends on the network speed to supply the next file in-time to play.
VOX starts to load next track several seconds before previous track ends playing, that's for battery reasons. If that time is not enough for network to start delivering next track - then gap occurs.
To make sure that gapless is working, try download Your album on Your device and listen from there.