IMO there should be one, unified way of getting rid of assertions in code, when compiling. Now it looks like this:

1) all assertions (implicit & explicitly declared by assert()) are compiled in by default
2) --disable-assert disables only implicit assertions
3) to disable assert(), you CAN'T DO THIS AT THE VALA LEVEL. You must pass "-X -DG_DISABLE_ASSERT", and assertions will be disabled by the C compiler.

Asssertions are commonly used to check constrains in debug versions, I know that Glib gives them slightly different role, but there should be some way of debug-version-only asserting.

A little offtopic:

My proposition would be even to introduce keyword "debug" with syntax:

debug {
//statements
}

And the debug block would be compiled into program only in debug builds.