For example:

using GLib;

public class Test : Object {
	delegate int Accumulates(int x, int acc);

	public void run() {
		Accumulates fac;

		fac = (x, acc) => {
			if (x <= 1)
				return acc;
			else
				return fac(x - 1, x * acc);
		};

		stdout.printf("Result is: %d\n", fac(5, 1));
	}

	public static void main(string[] args) {
		new Test().run();
	}
}

Vala compiles this fine, but the generated code has errors.

Although Vala probably won't support full closures, recursive lambdas probably shouldn't be too hard to do (in __lambda0 in the generated code, just replace fac_target -> self and fac -> ___lambda0_test_accumulates). I'll see if I can make a patch for this...