If by "compatible", you mean that it will work in a system with DX10 installed, yes
Note that, DX10 and DX9 are both installed at the same time, it's not like when upgrading from DX8 to DX9 or previous upgrades, were the older version disappeared, and the only DirectX on the system was the newest one (in DX9, older games works thanks to DX7/8 backward compatibility that DX9 offers).
In Vista, there are basically 3 different DirectX versions all available:
- Direct3D 9, that emulates DX9 as if it was running on XP. This is the only DX available if one is using XP drivers in Vista (for example, when updating from XP to Vista without reinstalling)
- DirectX 9.0L, that is a DX9 that can use new Vista drivers (WDDM). This is how FS9 works under Vista, if there's a native Vista driver installed.
- DirectX 10, the new DX, that requires both a native Vista WDDM driver, a compatible graphic card (must support shader model 4.0), AND an application specifically written to use it.
So, even if you are using native Vista drivers and a card that supports DX10, FS9 will never be able to use DX10, but it will work under the DX9 that is still available in Vista, like 95% of the current games.