A USB interface is definately an option of choice since you have USB available..MIDI doesnt take up much bandwidth either so it's not too much of an issue when sharing USB resources. Dont go thru a USB hub with MIDI tho since some data can be lost in the transmission. To sync up to VS880's make sure you have a digital cable between the two 880's making one the master and the second the slave. In Nuendo simply send MTC to the MIDI port feeding the first 880 and then daisey chain the Midi to the other 880..make sure to transmit MMC from the first 880 and make the second 880 the slave to MMC..your DPS should have a seperate Midi send from the USB device...Nuendo allows you to send multiple outputs a single MTC output. That should take care of that issue...as far as the O2R controlling nuendo you might want to look in the DEvice Setup area and go to the Add/Remove section and see if their is a template for it or not..I cant remember off the top of my head. If not I'm pretty sure there is a template floating around for it on the Nuendo forums.
Opus

The O2R is not supported as a controller device by Nuendo. You can get specific information about this by searching the http://www.nuendo.com support forums. Apparently the O1V was a lot easier for them to implement and now they are involved with their Houston Controller, soon to be released. There is a way to map some functions and again search the Nuendo Forums and you should be able to get some info about this.