Make sure vs-emulation--mode* uses the correct value of vs-emulation-mode.

If some Lisp code did:

(vs-emulation-mode 'toggle)
(vs-emulation-mode 'toggle)

vs-emulation--mode*, run inside an idle timer, would see the same value of
vs-emulation-mode twice, thus incorrectly enabling or disabling the mode
twice.
Use #' to quote function names.

This allows the byte compiler to warn when the code refers to an undefined
function.