Maybe ...

If the data/RS/E pins are not tri-stated while the power to the VFD is OFF, it could be trying to power it from those pins.
Then when the transistor turns on, it doesn't activate the power-on reset in the display.

When programming with the PICkit, those pins will always tri-state during programming.

Just turning off the transistor without tri-stating all the pins first may not be enough.
Then remember to clear the FLAGS variable, so PBP will re-initialize the display.