... I have it in the effects loop of my main Amp.
It just occurred to me that this may be the reason that the tuner is not working for you, despite your attempts using the known MASTER INPUT GAIN fix.
Ideally, your bass needs to be directly connected to the V-AMP first and then the V-AMP should feed the EFFECT RETURN jack on your main amp for the levels and signal quality to be what the TUNER is expecting to receive. You may want to try this as an option before purchasing an external tuner, at a minimum, simply to determine if there truly is an operational issue with your V-AMP.