As almost each unlocker know, VPP is used to boost flash erase and store time.
It's common used for older NOR phones (in latest NAND/ONENAND, they are fast enough without VPP).
Before flashing phone with VPP make sure your cable have VPP connected to PIN 8 (or pin 9 on 10pin layout or pin 3 with adapter).
Now there is 2 boosting options:

1. Box can either supply own, internal generated VPP signal, but only 5v. So you need attach VPP Pump, then 9V will appear on output, and flashing will be boosted.
2. Box can supply native VPP signal. But correct DC power supply needs be applied. 10v should be enough (input diode resulting in voltage drop - about 0,6V + power IC also dropping some, so keep note that), programming not taking a much current so 100-500mA will be very fine (of course more will not do any damage).
You can check output voltage by running box selftest, so you can ensure you will kill or not kill your flash chip. As long as power supply manufacturers sometime likes to make some jokes