install an acceptable version of my patch, as it improves the status
    quo, and include it in the next release. Then I'd think about
    whether it's possible to redo the Vala support entirely.

Sounds sensible.

I applied your change, but when running make distcheck (not make check)
the new vala-libs-distcheck.sh test fails. Does that happen for you?

Most of the other vala tests are skipped, with the message:
vala-libs-vpath: skipped test: pkg-config m4 macros not found