I think most would say that the relationship would go Positive intrathoracic pressure -> worsening of venous congestion -> worse venous Doppler (but still reliable reflect venous congestion)

VExUS was developed in ventilated patients (ICU admission after cardiac surgery) but they basically had very similar parameters.

Our Danish colleagues did the best study in my opinion, they modified body position and peep. The later did not change the venous Doppler indices much but they did not push it further than 12 mmHg

Effects of changes in position, positive end-expiratory pressure and mean arterial pressure on renal, portal and hepatic Doppler ultrasound perfusion indices: a randomized crossover study in cardiac surgery patients – PubMed