PRE will work on your raw bass signal first before going through to the AMPS, COMPRESSION + EFFECTS sections.
POST will work on the fully-completed bass tone processed via DSP, right before it hits the MASTER volume knob.
Personally, with 3 EQ points in the V-AMP, with the primary one being sweepable in the MID, I would find it hard to justify using an external EQ for my own needs. Additionally note that you will not be able to store your external EQ settings along with different patches as would occur if EQ'd solely inside the V-AMP.
With this said though, what do I know? This is your rig!
In all seriousness, you may find use for the money saved on an EQ to spend elsewhere?
Cheers!