Terrain Collision

Surely you just check the terrain of the tile (or polygon or whatever) that the unit would be on in the next frame using its current vector and if it is impassable then do not allow the move. Sounds like maybe a data-structure problem. Do you have efficient means to determine what tile is under a certain coordinate/position? If you do I can't see what the problem is. Each tile will know its own type, so you just check the type to see if it is impassable.
If you don't then you should create one (if the tiles are on a grid then it should simply be scaling the coordinate by a certain amount, and that will give you an index into the tile grid).

Your wording is slightly confusing and perhaps lacking in details, how you do this will be down to the specifics:

- How is your data organised? Is your terrain a grid with each cell knowing whether it is passable or not, or is the terrain a large heightmap field with other ways of defining "impassable"
- If it is a grid, how many tiles can a unit move each "move turn", if its one then you will simply be checking whether that tile is "impassable", however if its more then one you are going to have to run a path finding algorithm to determine if a path to that position is possible and then act accordingly.

Alternatively, I have failed and read your question completly wrong :p

Scott