Hi,
I don't have either Tacpac or P3D, so I cannot pinpoint the problem.
But if disabling TacPac solves the problem of not being able to do VTOL even with almost empty tanks:
It sounds like the aircraft is over-weighted.
Because I know Tacpac manipulates the weight of the aircraft, via the payload stations.
What I would like to know, if VTOL doesn't work with Tacpac enabled: what is the total weight of the aircraft then ?
You can check this in the menu, or use the tooltip in the top-left red/green led button in the VTOLdisplay gauge.
If total weight is more then 21000 lbs, you can't do a vertical takeoff.
Rob