I did said it use default clock, but the default clock might be different with a real PSP.
Based on what i found on some forum it seems Vita is emulating PSP-1000 model https://wololo.net/talk/viewtopic.php?t=49796
And based on DBZ Team Tag multiplayer, it's saying that PSP-1000 and PSP-2000 aren't compatible (at least for DBZ TT, which use different clock on different PSP model), so may be you need use PSP-1000 model for Burnout to be compatible with Vita?
Based on what i found on some forum it seems Vita is emulating PSP-1000 model https://wololo.net/talk/viewtopic.php?t=49796
And based on DBZ Team Tag multiplayer, it's saying that PSP-1000 and PSP-2000 aren't compatible (at least for DBZ TT, which use different clock on different PSP model), so may be you need use PSP-1000 model for Burnout to be compatible with Vita?
PPSSPP Multiplayer Guide: https://github.com/hrydgard/ppsspp/wiki/How-to-play-multiplayer-games-with-PPSSPP