I dont think vta is bad in this car as it isn't maf tuned.
VTA with a MAP sensor is ok if you really need to install a BOV, but when you run a BOV to atmosphere with a MAF sensor, the engine will run very rich for a short period of time after a gear up change because the intake air that was recirculated has now been vented. The ECU has been signalled by the MAF sensor to injected a set amount of fuel according to the volume of air measured by the MAF sensor. Because some of this air has been vented the engine will run rich and may even stall between gear shifts.
The MAP sensor is measuring volume in the intake manifold and can compensate for the loss of intake air when the BOV vents to atmosphere during up shifts.
The other consideration that is rarely mentioned is that during high manifold vacuum, for example driving down a hill with the throttle closed, the BOV is open and drawing in unfiltered air into the engine.