The VLPSA flag can be triggered when one of the peripherals, such as FlexCAN, fails to acknowledge the transition to VLPS mode.
It also appears that you are using very old drivers, likely the legacy SDK drivers, which are no longer supported. I strongly recommend migrating to the latest RTD drivers.
To debug this issue, open POWER_SYS_SetMode() and locate the WFI instruction in the corresponding sub‑function. Set a breakpoint at that point. When the core halts on the WFI, inspect all relevant clock and peripheral registers.
Regards,
Daniel