I have only played with the VDIP1, but I think they are pretty close.
Here is a good place to start with the VDIP
http://www.picbasic.co.uk/forum/showthread.php?t=7700

Not sure if the parallel idea will work or if it is necessary. All info in and out of the VDIPs goes through the on board control module. When reading date from a flash drive with the VDIP you can not stop it. It does not have a flow control that works that way. So I would suggest an echo routine using the PICs USART. Here is an example.
http://www.microengineeringlabs.com/.../pbp/usart.bas
You can use feed back from the VDIP to let the PIC know when it is finished sending data, see the first link. Send comands to the VDIP with SEROUT2 and recieve with the hardware port. Then send the data out from the PIC with the hardware port.

PIC selection... I do not think you can have to much ram. Sounds like you will need all you can get for the file name arrays. Most any of the 18s should do.