While trying to test a fix of a problem described in #721570 on branch wip/721570-smtp-auth, I found out there's no way to pass dynamically the position of vapigen to the build scripts:
On Gentoo, vala 0.22.1 installs vapigen as /usr/bin/vapigen-0.22


This is the output I get:

cmake --no-warn-unused-cli -C /var/tmp/portage/mail-client/geary-9999/work/geary-9999_build/gentoo_common_config.cmake -G Unix Makefiles -DCMAKE_INSTALL_PREFIX=/usr -DDESKTOP_UPDATE=OFF -DGSET
TINGS_COMPILE=OFF -DICON_UPDATE=OFF -DVALA_EXECUTABLE=/usr/bin/valac-0.22 -DWITH_UNITY=OFF -DDESKTOP_VALIDATE=OFF -DCMAKE_BUILD_TYPE=Gentoo -DCMAKE_INSTALL_DO_STRIP=OFF -DCMAKE_USER_MAKE_RULES
_OVERRIDE=/var/tmp/portage/mail-client/geary-9999/work/geary-9999_build/gentoo_rules.cmake  /var/tmp/portage/mail-client/geary-9999/work/geary-9999
Not searching for unused variables given on the command line.
loading initial cache file /var/tmp/portage/mail-client/geary-9999/work/geary-9999_build/gentoo_common_config.cmake
-- The C compiler identification is GNU 4.7.3
-- Check for working C compiler: /usr/bin/x86_64-pc-linux-gnu-gcc
-- Check for working C compiler: /usr/bin/x86_64-pc-linux-gnu-gcc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Found PkgConfig: /usr/bin/x86_64-pc-linux-gnu-pkg-config (found version "0.28") 
-- xgettext found
-- Found Vala: /usr/bin/valac-0.22  
-- checking for a minimum Vala version of 0.22.1
--   found Vala, version 0.22.1
CMake Error at src/CMakeLists.txt:404 (message):
  vapigen must be installed to build Geary.


-- Configuring incomplete, errors occurred!


I also noticed that the vapigen executable name is hardcoded both in src/CMakefile.txt and in bindings/vapi/Makefile