Having more than one VSD in a cockpit means that each VSD may have different settings.
For example, the copilots VSD range may be set to 50 nm, while the captain's VSD range may be set to 100 nm.
Basically, this means that several instances of the same instrument may need to work with different data at run time.
Just imagine a traditional ND or PFD/AI: There are pilot-specific settings available, which do not usually affect the other pilot.
Thus, using a dedicated location in the property tree ensures that each "version" (instance actually) of the instrument can work with its own "private settings".
If your VSD implementation is already part of the ND, then, I'd assume each ND has already its own VSD, too - right?
Browsing the property tree, you should see your VSD-specific properties show up in dedicated sub branches of each ND, right?
Regarding the corridor: Sampling a straight line from the aircraft doesn't really cater for safety requirements.
In other words, the aircraft is unlikely to actually "fly that particular straight line". That's why avionics use an averaged altitude (Minimum Safe Altitude actually) based on a "corridor" ahead of the aircraft.
Say, you'd define the corridor to be 0.5-1 nm wide, that would mean that the avionics would query the MSA for that corridor, and show the LOWEST altitude for that corridor.
Yes, that means more terrain sampling - so you may want to disregard this for the moment. But that's actually how it works in real life. The links I posted previously go into much greater detail, if you are interested in this.
Regarding TCAS, for your instrument you don't actually need to instantiate an actual TCAS at all. In its simplest form, you could simply "cheat" and directly process the properties under /ai/models, where you can find all AI and multiplayer aircraft. So, your Nasal script can iterate over this list and dynamically add symbols for those aircraft next to you.
A while ago, I wrote a short tutorial on this, and added it to the wiki:
http://wiki.flightgear.org/Howto:_Worki ... properties