Sergey,
Well you actually can force this with subsequent oddities showing up. If you have a VFP8 exe that makes a FoxPro style call to a compiled VFP9 exe the 9 code will be running under the 8 runtime. The errors and bugs that happen because of this are pretty obtuse too.
>Each VFP EXE has version of runtimes to use stored inside. See
http://www.berezniker.com/content/pages/visual-foxpro/retrieving-vfp-runtime-dll-name-required-exe-or-dll.
>IOW, VFP8 EXE will use VFP8 runtimes and VFP9 EXE will use VFP9 runtimes. You cannot force EXE to use other version runtime.